Introduce mac_biba_copy() and mac_mls_copy(), which conditionally

Introduce mac_biba_copy() and mac_mls_copy(), which conditionally
copy elements of one Biba or MLS label to another based on the flags
on the source label element.  Use this instead of
mac_{biba,mls}_{single,range}() to simplify the existing code, as
well as support partial label updates (we don't update if none is
requested).

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories