Another try at socklen_t: just check for it, and otherwise use int.

Another try at socklen_t: just check for it, and otherwise use int.
The HP manual says this will work on old HP/UX versions; I'm not sure
about other systems.  Possibly it will break on old BSD-derived
systems with 32-bit int, 64-bit size_t and no socklen_t, if there are
any such.