Addressing SF bug #643005, implement socket.inet_aton() using

inet_aton() rather than inet_addr() -- the latter is obsolete because
it has a problem: "255.255.255.255" is a valid address but
indistinguishable from an error.

(I'm not sure if inet_aton() exists everywhere -- in case it doesn't,
I've left the old code in with an #ifdef.)