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

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.