Swap hardcoded AF_INET(6) values for Python-provided values as they
differ between Darwin and Linux (30 vs 10 for AF_INET6 for instance).

When I starting sshuttle with option `--seed-hosts example.com`, got the following error:

```
hostwatch: Starting hostwatch with Python version 3.5.2
hostwatch: Traceback (most recent call last):
--->   File "sshuttle.server", line 144, in start_hostwatch
--->   File "sshuttle.hostwatch", line 272, in hw_main
--->   File "sshuttle.hostwatch", line 234, in check_host
--->   File "sshuttle.hostwatch", line 32, in _is_ip
--->   File "/usr/lib/python3.5/re.py", line 163, in match
--->     return _compile(pattern, flags).match(string)
---> TypeError: cannot use a string pattern on a bytes-like object
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "assembler.py", line 37, in <module>
  File "sshuttle.server", line 393, in main
  File "sshuttle.ssnet", line 596, in runonce
  File "sshuttle.server", line 324, in hostwatch_ready
sshuttle.helpers.Fatal: hostwatch process died
```

It seems like the list of hosts is not properly decoded on the server side. This is an attempt to fix that.

With this configuration it should be feasible to achieve a perfect score
without contortion.

Rules skiped for Bandit:
B101: assert_used
B104: hardcoded_bind_all_interfaces
B404: import_subprocess
B603: subprocess_without_shell_equals_true
B606: start_process_with_no_shell
B607: start_process_with_partial_path

Rules skiped for pylint:
- too-many-statements
- too-many-locals
- too-many-function-args
- too-many-arguments
- too-many-branches
- bare-except
- protected-access
- no-else-return

As suggested by @colinmkeith the UDP and DNS proxies should listen on different
ports otherwise the DNS proxy can get traffic intended to the UDP proxy (or
vice-versa) and handle it incorrectly as reported in #178.

At first sight it seems that we had the code in place to try another port if
the one we are binding is already bound, however, with UDP and REUSEADDR the
OS will not refuse to bind two sockets to the same socket address, so both
the UDP proxy and DNS proxy were being bound to the same pair.

Some Linux distros, like Alpine, Arch, etc and some BSDs, like FreeBSD, are
now shipping with python3.6 as the default python3. Both the client and the
server are failing to run in this distros, because we are specifically looking
for python3.5.

These changes make the run shell script use python3 if the version is greater
than 3.5, otherwise falling back as usual.

On the server any version of python3 will do, use it before falling back to
python, as the server code can run with any version of python3.

When the pf module is not loaded our calls to pfctl will fail with
unhelpful messages.
This change spares the user the pain of decrypting those messages and manually
enabling pf. It also keeps track if pf was loaded by sshuttle and unloads on
exit if that was the case.

Also fixed the case where both ipv4 and ipv6 anchors were added by sshuttle
but the first call of disable would disable pf before the second call had the
chance of cleaning it's anchor.

If we receive no routes from server or if, for some reason, we receive
some empty lines, we should skip them instead of crashing.

Fixes on of the problems in #147.

Currently hostwatch only adds hostnames even when FQDNs are available.
This commit changes found_host so that when the name is a FQDN, both the FQDN
and an hostname are added, e.g., given api.foo.com both api and api.foo.com
will be added.

Fixes #151 if merged.

N.B.: I rarely use hostwatch, it would probably be a good idea to get feedback
from people who actually use it before merging. Not too sure about this...

While with AF_INET sockaddr is a 2-tuple composed by (address, port),
with AF_INET6 it is a 4-tuple with (address, port, flow info, scope id).

We were always passing a 2-tuple to socket.connect which would fail whenever
the address was, for instance, a link-local IPv6 address that needs a scope id.

With this change we now use getaddrinfo to correctly compute the full tuple.

Fixes #156.

Per https://github.com/apenwarr/sshuttle/pull/45/files

When doing port forwarding on lo0 avoid the special case where the
traffic on lo0 did not came from sshuttle pass out rule but from the lo0
address itself. Fixes #159.