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

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.