Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

o style.Makefile(5): WARNS= -> WARNS?=.

Style: NO_MAN doesn't need any value.

Add a regression test to test two variations on the same bug: joining a
multicast group using a raw socket, then removing the interface on which
the group is found, and joining a multicast group using a udp socket,
then removing the interface on which the group is found. An if_disc
interface is used as the interface on which to attach.

NB: A panic currently results from running this regression test, so do
so with caution.

PR: 77665
Reported by: Gavin Atkinson <gavin dot atkinson at ury dot york dot ac dot uk>
Reported by: Brooks Davis <brooks at FreeBSD dot org>