unix socket: Store module constants in data structure.

-147 bytes on x86.
unix socket: Add few more socket constants.
Add MP_OBJ_IS_INT(), for symmetry with MP_OBJ_IS_STR().
Functions of fixed number of args are special-cased only for 3 or less args.
unix socket: Add setsockopt() method.
sock-server.py: Use SO_REUSEADDR to avoid errors on quick restart.