tcp socks proxy support

[totaam]

Now that we have ssh proxy support (#2041), adding socks proxy support would be much easier.
This may even be auto-configured using libproxy.

See also network tracker ticket: #1590

[totaam]

For win32 support, we may need to revisit #2127#comment:5 and r21648.

[totaam]

With the last commit, system proxy settings aren't used by default but it's easy enough to enable:

xpra attach "tcp://hostname:10000/?proxy=auto"