litex_server: check socket flags exist before using them

litex_server: check socket flags exist before using them

Some flags are only available on certain platforms.  Verify these flags
exist prior to using them when opening a socket.

See
https://stackoverflow.com/questions/14388706/socket-options-so-reuseaddr-and-so-reuseport-how-do-they-differ-do-they-mean-t
for more information

Signed-off-by: Sean Cross <sean@xobs.io>