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

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

Remove last remnants of classful addressing:

- Remove ia_net, ia_netmask, ia_netbroadcast from struct in_ifaddr.
- Remove net.inet.ip.subnetsarelocal, I bet no one need it in 2011.
- fix bug when we were not forwarding to a host which matches classful
net address. For example router having 192.168.x.y/16 network attached,
would not forward traffic to 192.168.*.0, which are legal IPs in
CIDR world.
- For compatibility, leave autoguessing of mask based on class.

Reviewed by: andre, bz, rwatson

Add initial inet DDB support for show in_ifaddr and show sin commands which
proved to be useful while debugging address list problems.

MFC after: 6 days