Include netinet/in_systm.h in lm-sock.c, fixes LM-130. Added check for netinet/in_systm.h and include it in lm-sock.c if it exists. It fixes a build error on BSD.

Colin Walters
Frederic Crozat
Josh Beam
Martyn Russell
Richard Hult
Ross Burton
Sjoerd Simons
Tim Robbins
Tollef Fog Heen