support automatic port allocation

[Antoine Martin]

So we can do:

xpra --bind-tcp=0.0.0.0:0

And let the OS choose a port number of us.
Print it out and expose it via xpra info.

[Antoine Martin]

Related to r20286.
Done in r20287.

ie:

$ xpra start --start=xterm --bind-tcp=0.0.0.0:0 --no-daemon  --bind-ws=0.0.0.0:0 
allocated tcp port 35543 on 0.0.0.0
allocated ws port 37307 on 0.0.0.0
(..)

This works with all socket types, including UDP, and the correct port number is shown in the mdns session browser.

@maxmylyn: just a FYI.

[J. Max Mena]

Noted and closing.