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

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

Fix alignment related warnings reported by gcc
Switch from old style arguments to modern C
remove WARNS as a result

tested with "make universe"

Approved by: cperciva
MFC after: 1 week

The last big commit: let usr.sbin/ use WARNS=6 by default.

add ip6addrctl(8) which is a command to configure address
selection policy for IPv6 and IPv4 described in RFC3484.
source address selection part of RFC3484 is not merged
from KAME, yet.

Obtained from: KAME