poll(2): Add POLLRDHUP.

Teach poll(2) to support Linux-style POLLRDHUP events for sockets, if
requested.  Triggered when the remote peer shuts down writing or closes
its end.

Reviewed by:	kib
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D29757

newlib/libc/sys/rtems/include/sys/poll.h

@@ -71,6 +71,7 @@ struct pollfd {
 #if __BSD_VISIBLE
 /* General FreeBSD extension (currently only supported for sockets): */
 #define	POLLINIGNEOF	0x2000		/* like POLLIN, except ignore EOF */
+#define	POLLRDHUP	0x4000		/* half shut down */
 #endif
 
 /*