Prefer SOCK_NONBLOCK to O_NONBLOCK

Prefer SOCK_NONBLOCK to O_NONBLOCK

* src/process.c (SOCK_NONBLOCK): Define to 0 if not already defined.
(connect_network_socket): Create the socket with SOCK_NONBLOCK, to
avoid an fcntl with O_NONBLOCK if SOCK_NONBLOCK works.  Put the
SOCK_DGRAM check a bit later, to keep the logic cleaner, as
the order does not matter here.