emacsclient: assume HAVE_INET_SOCKETS

emacsclient: assume HAVE_INET_SOCKETS

* configure.ac (HAVE_INET_SOCKETS): Remove.
* lib-src/emacsclient.c: Simplify by assuming HAVE_SOCKETS and
HAVE_INET_SOCKETS, which are always true nowadays, except perhaps
for MS-DOS and if so this program shouldn’t be built there anyway.
Don’t bother including sys/types.h, as it’s not needed on modern
systems (and syswait.h does it for us anyway).
(main): Simplify by assuming SIGSTOP (which is always defined
if SIGCONT is), and by assuming HAVE_SOCKETS && HAVE_INET_SOCKETS.