chcon: exit immediately if SELinux is disabled

chcon: exit immediately if SELinux is disabled

This change happens to avoid an abort in chcon when SELinux is
disabled while operating on a file with an "unlabeled" context from
back in 2006.  However, that same abort can still be triggered by the
same file when running chcon with SELinux enabled.  This bug in chcon
will be fixed in a subsequent commit via a getfilecon wrapper.  See
http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/18378/focus=18384
for how to correct your disk attributes to avoid triggering this bug.
* src/chcon.c (main): Exit immediately if SELinux is disabled.
Reported in http://bugzilla.redhat.com/527142 by Yanko Kaneti.
* src/runcon.c (main): Do not hardcode program name in error message.
* THANKS: Update.