This change switches the use of socket implementation from the
conventional SUS-based one to the latest POSIX-based one to make
socket control message work correctly on Solaris.

It looks like those two implementations, Socket over TLI/XTI and
Socket, have different semantics in details but it wouldn't hurt
the existing applications because the exposed syscall API doesn't
support socket properties related to such a protocol independent
application framework.

Fixes #7402.

Change-Id: I45a4e782d606bfbebe1404086c50a8c69af53461
Reviewed-on: https://go-review.googlesource.com/30171Reviewed-by: Ian Lance Taylor <iant@golang.org>