Ignore selevel/serange if MLS is disabled

Currently, the selevel/serange values (which are often set on a default
's0' value) are used for ports, users, contexts and logins. This breaks
non-MLS setups.

This patch will only call the necessary mls functions if mls is actually
enabled.

Signed-off-by: Sven Vermeulen <sven.vermeulen@siphos.be>
policycoreutils 2.2.5 - yet another bug fix for non-MLS systems.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>