getsockopt.2: Add SO_SPLICE source socket read and socket close behavior

Add select(2), poll(2), or kqueue(2) source socket semantics and close
behavior.

Reported by: Udip Pant <upant@netflix.com>
Reviewed by: manpages (ziaee), markj
Obtained from: OpenBSD
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D51547

(cherry picked from commit 122b2a3530f8baab56d196b51724cc3ddd38abd2)