Add support for listening on a UNIX domain socket

This enabled the OPA server to listen on a UNIX domain socket through
the already existing --addr option. This option now parses the passed
argument as a URL and based on the scheme (which for this case has to be
"unix://"), it listens on the path for the socket.

Signed-off-by: Juan Antonio Osorio Robles <jaosorior@redhat.com>