Introduce support for Mandatory Access Control and extensible

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

Invoke an appropriate MAC entry point to authorize execution of
a file by a process.  The check is placed slightly differently
than it appears in the trustedbsd_mac tree so that it prevents
a little more information leakage about the target of the execve()
operation.

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