Introduce support for Mandatory Access Control and extensible

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

Hook up various policy modules to the kernel build.

Note that a number of these modules require futher entry point commits
in the remainder of the kernel to become fully functional, but enough
of the pieces are in place to allow experimentation.

Note also that it would be desirable to not build the mac_*.ko modules
if 'options MAC' is not defined in the kernel configuration, because
the resulting modules are not useful without the kernel option.  There
doesn't appear to be precedent for a way to do this -- for example,
we allow ipfw.ko to be built even if 'options NETINET' isn't defined.
Suggests welcomed on the "best" way to do this.

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