IPv6 support for OSX and BSDs

Adds IPv6 support for OpenBSD and OSX.

AF_INET is the same constant on Linux and BSD but AF_INET6
is different. As the client and server can be running on
different platforms we can not just set the socket family
to what comes in the wire.

We were always excluding 127.0.0.1/8 but sshuttle might be listening on
other IP, e.g., ::1 for IPv6 or any other defined with -l

Should fix issue #104.

Add <forward_subnets> to divert rule in OpenBSD

Fixes bug where all traffic routed to loopback would end up being diverted to the same port.

A possible implementation for the change requested in #94, so that seed
hosts can be used without auto hosts. In this scenario only the
specified hosts (or ips) will be looked up (or rev looked up).

Otherwise the output can vary and confuse our attempts to parse it.

Fixes: 93

We shouldn't come up with a fatal error because of a ENETUNREACH when
trying to contact the DNS server. Although this error shouldn't happen
either.

Fixes #89.

Sometime ago I was in python mode and incorrectly indented a line of the
shell script with spaces instead of tabs. Shame on me. This should bring
things back to their natural order.

Previously the sshuttle shell script would pass the python to use as the
first argument of the command. The new run script no longer does this.
Instead we can obtain the python being used via sys.executable.
Fixes #88.

readthedocs alters docs/conf.py which in turn means python_scm detects a
version and incorrectly adjusts the version number. Here we try to work
around this problem.

We do this by renaming the docs/conf.py file and copying it back to
docs/conf.py when setup.py is invoked. This way, hopefully, scm won't
see the changes to docs/conf.py

References:
http://stackoverflow.com/questions/35811267/readthedocs-and-setuptools-scm-version-wrong/36386177
https://github.com/pypa/setuptools_scm/issues/84

This reverts commit 7ea2d973.

In preparation for new release.

It is often the case that the user has no administrative control over
the server that is being used. As such it is important to support as
many versions as possible, at least on the remote server end. These
fixes will allow sshuttle to be used with servers that have only
python 2.4 or python 2.6 installed while hopefully not breaking the
compatibility with 2.7 and 3.5.

When passing multiple subnet files, e.g., by using -s/--subnets
multiple times or by using it together with subnets passed as positional
arguments append the content from all sources instead of only using the
subnets from the last source. This makes the behaviour of -s/--subnets
consistent with -x/--exclude.

As @brianmay observed in #82 this option is no longer used and can be
dropped.

Fixes the kind of problems reported on #75 but does break the command
line "API" (hopefully).