(FACT-647) Cherry-pick lost SELinux exception handling back into master

This commit cherry-picks work done on the selinux facts
back into master, which was lost during the facter-2 / master merge.

Original commit(40603c9):
Catch exceptions when detecting SELinux status
Merge pull request #754 from whopper/selinux-cherrypick

(FACT-647) Cherry-pick lost SELinux exception handling back into master