Introduce support for Mandatory Access Control and extensible

Introduce support for Mandatory Access Control and extensible
kernel access control.

When fragmenting an IP datagram, invoke an appropriate MAC entry
point so that MAC labels may be copied (...) to the individual
IP fragment mbufs by MAC policies.

When IP options are inserted into an IP datagram when leaving a
host, preserve the label if we need to reallocate the mbuf for
alignment or size reasons.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, NAI Labs