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

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

Add infrastructure to allow all frames/packets received on an interface
to be assigned to a non-default FIB instance.

You may need to recompile world or ports due to the change of struct ifnet.

Submitted by: cjsp
Submitted by: Alexander V. Chernikov (melifaro ipfw.ru)
(original versions)
Reviewed by: julian
Reviewed by: Alexander V. Chernikov (melifaro ipfw.ru)
MFC after: 2 weeks
X-MFC: use spare in struct ifnet

Factor out DDB commands from r204145, r204279 into if_debug.c for further
enhancements (1). Switch to a standard 2-clause BSD license for this (2).

Unfortunately we have to un-static the ifindex_table for this but do not
publicly export it.

Suggested by: rwatson (1) a while back.
Approved by: thompsa (2) for the change from r204279.
MFC after: 6 days