per socket authentication and encryption configuration

[Antoine Martin]

Split from #1159.
We should extend the socket configuration syntax to be able to configure authentication and encryption for each socket.
This would get rid of a number of command line arguments (which we would keep temporarily for backwards compatibility, probably also still used by the client command line).

ie: for just authentication (replacing the tcp-auth command line option):

xpra start \
    --bind-tcp=0.0.0.0:10000,auth=file:filename=password.txt

for encryption as well:

xpra start \
    --bind-tcp=0.0.0.0:10000,encryption=AES:keyfile=key.txt,auth=file:filename=password.txt