IPv6 support in nft method.

Update docs to indicate that IPv6 is supported with the nft method.

- Adds nft into the requirements.rst file.

- Update description of what happens when a hostname is used in a
  subnet.

- Add ipfw to list of methods.

- Indicate that --auto-nets does not work with IPv6. Previously this
  was only mentioned in tproxy.rst

- Clarify that we try to use "python3" on the server before trying
  "python".

This works for me but needs testing by others. Remember to specify a
::0/0 subnet or similar to route IPv6 through sshuttle.

I'm adding this to nft before nat since it is not sshuttle's default
method on Linux. Documentation updates may be required too.

This patch uses the ipaddress module, but that appears to be included
since Python 3.3.

Make nat and nft rules consistent; improve rule ordering.

First, check if TTL indicates we should ignore packet (instead of
checking in multiple rules later). Also, nft method didn't do this at
all. Now, nft matches the behavior of nat.

Second, forward DNS traffic (we may need to intercept traffic to
localhost if a DNS server is running on localhost).

Third, ignore any local traffic packets. (Previously, we ignored local
traffic except DNS and then had the DNS rules). The nft method didn't
do this previously at all. It now matches the behavior of nat.

Lastly, list the subnets to redirect and/or exclude. This step is left
unchanged. Excluding the local port that we are listening on is
redundant with the third step, but should cause no harm.

In summary, this ordering simplifies the rules in nat and eliminates
differences that previously existed between nat and nft.

Allow no remote to work.

Make prefixes in verbose output more consistent.

Use 'c' prefix for client, 's' prefix for server, and 'fw' prefix for
firewall messages. The 'c' and 's' prefixes were used sometimes but
not consistently. The firewall printed messages prefixed with
"firewall manager:" or "firewall:" or ">>" previously.

This patch also fixes a couple of print() calls that should have been
debug1()---a bug introduced in a recent commit.

Document -s/--subnets option in man page

When subnets and excludes are specified with hostnames, use all IPs.

Only write /etc/hosts when necessary.

Without this patch, sshuttle 'restores' /etc/hosts even if it didn't
make any modifications to it. This can be confirmed by running without
--auto-hosts and confirming that the modification time of /etc/hosts
is unchanged while sshuttle is running, but is updated when sshuttle
exits (and a debug2() message is printed indicating the file is
written).

I'm not aware of the previous behavior causing problems. However,
writing an important file unnecessarily as root should be avoided.

Pull request #502 made -r/--remote required. However, the
documentation still indicates that using no remote is a valid way to
test sshuttle (see Examples section of man page). I think this mode
might be useful for testing performance local without ssh, local with
ssh, and remote with ssh.

This patch adds a warning when -r/--remote is missing but restores the
previous behavior.

sdnotify.py documentation

Additional comments, checks, warning messages, and diagnostic
information is printed out when the client starts.

We assume IPv4 is always present and enabled. We assume IPv6 is not
supported when it is disabled at the command line or when it is not
supported by the firewall method. Warn if IPv6 is disabled but the
user specified IPv6 subnets, IPv6 DNS servers, or IPv6 excludes that
are effectively ignored.

Instead of indicating which features are on/off, we also indicate if
features are available in the verbose output.

We also more clearly print the subnets that we forward, excludes, and
any redirected DNS servers to the terminal output.

These changes should help handling bug reports and make it clearer to
users what is happening. It should also make it more graceful when a
user specifies a subnet/exclude with hostname that resolves to both
IPv4 and IPv6 (but IPv6 is disabled in sshuttle).

The list of subnets to route over VPN and the list of subnets to
exclude are parsed in option.py parse_subnetport(). Hostnames or IP
addresses are supported. If a hostname was provided, only the first IP
address was considered. This could result in some traffic not
traversing the VPN that the user might expect should traverse it from
the arguments passed to sshuttle.

This patch makes the function handle all of the IPs if a hostname is
provided. If a user provides a hostname with a CIDR mask, problems can
occur and we warn the user about the issue.

If the user includes a hostname with both an IPv4 and an IPv6 address,
and the underlying method doesn't support IPv6, then this patch will
cause sshuttle to fail. I plan to provide a future patch where failure
won't occur if the only place IPv6 addresses appear is in the exclude
list. In that case it should be safe to ignore the IPv6 address.

This patch also changes parse_ipport() which is used by the --to-ns
option. If the user provides a hostname here, we just use the first IP
from the hostname and warn the user that only one is being used.

Include sshuttle version in verbose output.

Some bug reports include verbose sshuttle output but lack the version
that is being used. Including the sshuttle version in the output may
make it easier to handle future bug reports.

Add psutil as dependency in setup.py

Bump flake8 from 3.8.3 to 3.8.4

Bump pytest from 6.1.0 to 6.1.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.3 to 3.8.4.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.3...3.8.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [pytest](https://github.com/pytest-dev/pytest) from 6.1.0 to 6.1.1.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/6.1.0...6.1.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump pytest from 6.0.2 to 6.1.0

Bumps [pytest](https://github.com/pytest-dev/pytest) from 6.0.2 to 6.1.0.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/6.0.2...6.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump pytest from 6.0.1 to 6.0.2

Bumps [pytest](https://github.com/pytest-dev/pytest) from 6.0.1 to 6.0.2.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/6.0.1...6.0.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Fix #494 sshuttle caught in infinite select() loop.

Bump attrs from 20.1.0 to 20.2.0