unix: add Poll function on FreeBSD

Tested with TestPoll extracted from syscall_linux_test.go. Once Poll is
supported on all OSes this test can be moved to syscall_unix_test.go.

Change-Id: I56ee0124a04c64e3ea51a1653901f2f4277983e6
Reviewed-on: https://go-review.googlesource.com/73370
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_freebsd.go

@@ -391,6 +391,15 @@ func IoctlGetTermios(fd int, req uint) (*Termios, 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
  */