BACKPORT: Smack: IPv6 host labeling

BACKPORT: Smack: IPv6 host labeling

IPv6 appears to be (finally) coming of age with the
influx of autonomous devices. In support of this, add
the ability to associate a Smack label with IPv6 addresses.

This patch also cleans up some of the conditional
compilation associated with the introduction of
secmark processing. It's now more obvious which bit
of code goes with which feature.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
(cherry-picked from upstream 21abb1ec414c75abe32c3854848ff30e2b4a6113)