Update UNIX domain socket support

These changes tweak the UNIX domain socket to (1) return an error
instead of panicing (because the server may be embedded as a library)
and (2) to unlink the domain socket file before binding. The latter is
required so that OPA can be stopped and started without manually
removing the socket file.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>