There's a known issue that makes sshuttle crash if there are too
many routes on the remote host (that don't fit in 64KB). This patch
requests the routes only if auto-nets is specified on the command
line.

This was susceptible to the same deadlock issue that ipt_chain_exists
had and was fixed in d43db80d where if the command returned a significant
amount of output, it wouldn't all be read in, resulting in the
subprocess hanging waiting for the output to be read.

subprocess.run only exists for python3, and this needs to also support
python 2.7

--ns-hosts is available since commit d2ee34d7
("dns: Added --ns-hosts to tunnel only some requests")
(released as v0.72), but was never documented.

--to-ns is available since commit be559fc7
("Fix case where there is no --dns.") after several
bugfixes, released as v0.78.4, but was never
documented.

When running sshuttle with a large list of routes it's failing to clean
them up at exit. It returns the following:

$ sshuttle -r user@host.example.com -s /tmp/aws-cidrs.txt
user@host.example.com's password:
client: Connected.
^CAnother app is currently holding the xtables lock; still -9s 0us time ahead to have a chance to grab the lock...
Another app is currently holding the xtables lock; still -19s 0us time ahead to have a chance to grab the lock...
Another app is currently holding the xtables lock; still -29s 0us time ahead to have a chance to grab the lock...

This continues indefinitely. Looking in ps reveals that there are 2
iptables processes running. Killing -9 the first one, allows sshuttle to
continue and clean up successfully.

The problem lies with the use of Popen here. The function currently
returns as soon as it finds a match without consuming everything from
stdout. This means that if there's more output from iptables than will
fit in the buffer it doesn't exit, and therefore doesn't release the
kernel xtables lock.

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.