Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

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.