per socket options

[Antoine Martin]

Based on #1160, we can re-use the same generic socket properties to configure more obscure socket settings like TCP_NODELAY (#619) and TCP_CORK (#2130).

[Antoine Martin]

r23896 does this for TCP_CORK and TCP_NODELAY, ie:

xpra start --start=xterm :10 --bind-tcp=0.0.0.0:10000,auth=file:filename=password.txt,cork=0,nodelay=1

Other things we can use this for:

• mdns=off
• ssl upgrade options
• limit the types of connections we can accept? (ie: info+version only)

[Antoine Martin]

r23904 makes it possible to disable mdns on a per-socket basis, ie:

xpra start --no-daemon --bind-tcp=0.0.0.0:10000,mdns=no --bind-tcp=0.0.0.0:10001,mdns=yes

SSL wrapping is more difficult because it takes so many arguments, and we're not currently storing them but taking them from the config object directly.
So they would need to be copied as attributes in the server object so we can re-use them later.