unix: add Poll function on Solaris

Tested with TestPoll extracted from syscall_linux_test.go.

Now that Poll is supported on all unix flavors, TestPoll can be moved
to syscall_unix_test.go in a successive CL.

Change-Id: Ibd5ff3f45198008464522c3b165f2af7865ce81e
Reviewed-on: https://go-review.googlesource.com/73881
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_solaris.go

@@ -578,6 +578,15 @@ func IoctlGetTermio(fd int, req uint) (*Termio, error) {
 	return &value, err
 }
 
+//sys   poll(fds *PollFd, nfds int, timeout int) (n int, err error)
+
+func Poll(fds []PollFd, timeout int) (n int, err error) {
+	if len(fds) == 0 {
+		return poll(nil, 0, timeout)
+	}
+	return poll(&fds[0], len(fds), timeout)
+}
+
 /*
  * Exposed directly
  */