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

Invoke the necessary MAC entry points to maintain labels on
process credentials. In particular, invoke entry points for
the initialization and destruction of struct ucred, the copying
of struct ucred, and permit the initial labels to be set for
both process 0 (parent of all kernel processes) and process 1
(parent of all user processes).

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