Before this change, in pf, exclusions used a pass out quick which gave
them higher precedence than any other rule independent of subnet width.
As reported in #265 this causes exclusion from one instance of sshuttle
to also take effect on other instances because quick aborts the
evaluation of rules across all anchors.

This commit changes the precedence of rules so quick can now be
dropped. The new order is defined by the following rule, from
subnet_weight:

"We need to go from smaller, more specific, port ranges, to larger,
less-specific, port ranges. At each level, we order by subnet
width, from most-specific subnets (largest swidth) to
least-specific. On ties, excludes come first."

* Close stdin, stdout, and stderr when using syslog or forking to daemon

Fixes #139

* Ensure we close devnull after use

* Fixes support for OpenBSD (6.1+)

As reported in #219, new versions of OpenBSD ship with a different
pfioc_rule struct. This commit adjusts the offset to match the new struct.

* Fixes tests for OpenBSD 6.1+

Having the tests in a `tests` directory in root is the most common
approach. Also moved pytest's conftest.py into `tests` making the
fixture available for client and server tests.

Add --no-sudo-pythonpath option

This provides a way to avoid setting PYTHONPATH when invoking the
privileged part of sshuttle with sudo. This is useful if running
sshuttle as a PEX archive, as Telepresence does, as it enables
sshuttle's sudo access to be securely locked down.

PEX archives will extract themselves into the invoking user's home
directory, which means that the invoking user has full control over
the code in them. This makes restricting sudo access with
PYTHONPATH set completely pointless in this scenario -- an attacker
could put any code into ~/.pex and gain full root access anyway.

On the other hand, if sshuttle is a PEX archive, the privileged
invocation will simply extract itself into /root/.pex anyway, so
there is no need to set PYTHONPATH in this case.

* works on ChromeOS with Crostini VM

tested on ASUS C101PA on Dev channel, should also work on Intel machines and Beta channel

* crostini doc, and a note about xterms and VNC

tested on ASUS C101PA on Dev channel, should also work on Intel machines and Beta channel

Peername is only used for information display messages.

Fixes #259

Remove reference to autossh per
https://github.com/sshuttle/sshuttle/issues/143

Fixes #227

Fixes #213

Automatic python 2.6 testing is becoming harder, especially as pytest
3.4.2 is unavailable for Python 2.6.

Starting sshuttle without having to type in one's password requires to
put the sudo-ed command in the `/etc/sudoers` file. However, sshuttle
sets an environment variable, which cannot be done as-is in the sudoers
file. This fix prepend the /usr/bin/env command, which allows one to
pass fixed environment variables to a sudo-ed command.

In practice, the sub-command:

```
sudo PYTHONPATH=/usr/lib/python3/dist-packages -- \
        /usr/bin/python3 /usr/bin/sshuttle --method auto --firewall
```

becomes

```
sudo /usr/bin/env PYTHONPATH=/usr/lib/python3/dist-packages \
        /usr/bin/python3 /usr/bin/sshuttle --method auto --firewall
```

This small change will allow a file path to be passed as argument from which
the command line options will be loaded.

Extra command line options can be passed (in addition to those already in the
file) and existing ones can be overriden.

Example sshuttle.conf file:
192.168.0.0/16
--remote
user@example.com

Example sshuttle call:
sshuttle @/path/to/sshuttle.conf

Example sshuttle call with verbose flags added:
sshuttle @/path/to/sshuttle.conf -vvv

Example sshuttle call overriding the remote server:
sshuttle @/path/to/sshuttle.conf -r otheruser@test.example.com