Introduce support for Mandatory Access Control and extensible

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

Instrument UFS to support per-inode MAC labels.  In particular,
invoke MAC framework entry points for generically supporting the
backing of MAC labels into extended attributes.  This ends up
introducing new vnode operation vector entries point at the MAC
framework entry points, as well as some explicit entry point
invocations for file and directory creation events so that the
MAC framework can push labels to disk before the directory names
become persistent (this will work better once EAs in UFS2 are
hooked into soft updates).  The generic EA MAC entry points
support executing with the file system in either single label
or multilabel operation, and will fall back to the mount label
if multilabel is not specified at mount-time.

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