per socket ssl options

[Antoine Martin]

Split from #2424: 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.

[Antoine Martin]

Client side done in r25186 + r25190 + r25191.

ie:

xpra attach "ssl://localhost:10000/?ssl-server-verify-mode=none&ssl-protocol=SSLv23"

[Antoine Martin]

Done in r25197.

Examples for server side:

xpra start --start=xterm --bind-tcp=0.0.0.0:10000,ssl-cert=./ssl-cert.pem,ssl-protocol=SSLv23 --bind-ssl=0.0.0.0:10001,ssl=cert=./ssl-cert2.pem

ie: each socket will use a different cert.

All the usual ssl options can be specified individually for each socket.
The existing --ssl-XYZ command line options are still valid and are used as default values.

For ssh, see #2583

[Antoine Martin]

Better syntax proposal in #2794