unix: add POLLRDHUP to FreeBSD

POLLRDHUP is a poll(2) event applicable to stream sockets that notifies
when a remote peer closes a connection.

Change-Id: I03a192426f89bbd08fb27dda2716ffb888043c92
Reviewed-on: https://go-review.googlesource.com/c/sys/+/607797
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: elindsey

unix/types_freebsd.go

@@ -356,6 +356,7 @@ const (
 	POLLRDNORM   = C.POLLRDNORM
 	POLLWRBAND   = C.POLLWRBAND
 	POLLWRNORM   = C.POLLWRNORM
+	POLLRDHUP    = C.POLLRDHUP
 )
 
 // Capabilities