Introduce support for Mandatory Access Control and extensible

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

Instrument devfs to support per-dirent MAC labels.  In particular,
invoke MAC framework when devfs directory entries are instantiated
due to make_dev() and related calls, and invoke the MAC framework
when vnodes are instantiated from these directory entries.  Implement
vop_setlabel() for devfs, which pushes the label update into the
devfs directory entry for semi-persistant store.  This permits the MAC
framework to assign labels to devices and directories as they are
instantiated, and export access control information via devfs vnodes.

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