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
(cherry picked from commit 3aaaa2efde896e19d229ee2cf09fe7e6ab0fbf6e)