chcon: don't disable just because SELinux is disabled

chcon: don't disable just because SELinux is disabled

* src/chcon.c (main): Now that gnulib provides getfilecon wrappers,
we can revert most of the 2009-10-05 commit 3a97d664, "chcon: exit
immediately if SELinux is disabled", since chcon is still useful as
long as the file system provides handlers for the security.*
name space.  gnulib's getfilecon wrappers ensure that an offending
context now evokes a return value of -1.
Prompted by comments from Stephen Smalley in
http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/18378/focus=18394
* NEWS (Bug fixes): Mention it.