syscall: fix Send{msg,msgN}, Recvmsg and control message handling on solaris

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/30171
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/syscall_unix_test.go

@@ -125,12 +125,6 @@ func TestFcntlFlock(t *testing.T) {
 // "-test.run=^TestPassFD$" and an environment variable used to signal
 // that the test should become the child process instead.
 func TestPassFD(t *testing.T) {
-	switch runtime.GOOS {
-	case "solaris":
-		// TODO(aram): Figure out why ReadMsgUnix is returning empty message.
-		t.Skip("skipping test on solaris, see issue 7402")
-	}
-
 	testenv.MustHaveExec(t)
 
 	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {