When packets pass in and out of six-to-four (STF) tunnels, perform

When packets pass in and out of six-to-four (STF) tunnels, perform
labeling checks and operations as with other network interfaces.
Eventually, if it proves desirable, we might want to offer special
casing of this or other tunnel interfaces where we have an existing
label of interest, rather than treating it as though it's an
entirely fresh mbuf in the incoming/outgoing encapsulation directions.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories