sockpair cleanup including:

provide prototype in lutil.h
use LBER_SOCKET_T instead of int in prototype.  (didn't use
	ber_socket_t so all of lber.h wouldn't have be dragged in)
rename signal handlers, use RETSIGTYPE, and make SIGHUP wake
rename fd to sd to reflect descriptor must be usable as a socket.
renamed sel_exit_fds to wake_sds to reflect current use
improve readability of sockpeer.c including removing obtuse goto.