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

Remove extra comment that snuck in in last commit.

Add a simple ICMPv6 filter test for IPv6 raw sockets: determine that
the default ICMPv6 filter is pass all, test that we can set it to block
all and restore to pass all. No attempt is made to test that the
filtering works, just that we can get and set it.