support starting new sessions with paramiko ssh proxy server

[Antoine Martin]

Starting a proxy:

xpra proxy :100 --bind-tcp=0.0.0.0:10000,auth=allow --no-daemon -d auth,ssh

Then trying to start a new session via this proxy:

xpra start ssh://localhost:10000/10 --start=xterm -d ssh

Fails with:

Warning: unsupported xpra subcommand '_proxy_start'

There's no reason why we can't allow that, even if it is optional (as a socket option?).

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2898