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"

[totaam]

This should be added to the documentation, until then, here is an example of manual proxy configuration (see above for auto):

xpra attach
    "tcp://host1:10000/?proxy-host=host&proxy-port=1080&proxy-rdns=no&proxy-username=username&proxy-password=password"

There are default values for all attributes except proxy-host:

xpra/xpra/scripts/main.py

Lines 920 to 925 in 811ae11

	host = to.strget("proxy-host")
	port = to.intget("proxy-port", 1080)
	rdns = to.boolget("proxy-rdns", True)
	username = options.get("proxy-username")
	password = options.get("proxy-password")
	timeout = options.get("timeout", 20)

[totaam]

If we ever want to move away from PySocks (no commits in 5 years.. missing SSL support, etc), there aren't many good options out there.
socks5's last commit was 8 years ago!