Place more stringent checks on process credential relabeling for the Biba

Place more stringent checks on process credential relabeling for the Biba
and MLS policies: as we support both an effective (single) element and
range (available) elements, require that the single be in the range if
both the single and range are defined in the update.  Remove comments
suggesting that such a check might be a good idea.

Don't introduce a similar check for network interfaces; due to different
interpretations of the single and range elements, it's not clear that
it's useful to do so.

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