Update tools/regression/poll/sockpoll.c for POLLRDPOLL.

Add a POLLRDHUP example to this tool, for comparison with other
operating systems. Also record current output on FreeBSD and Linux.

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

(cherry picked from commit 18f21f0355481283ceef0ec10e99554f44c205c2)