Ensure that we check for the special value CONFIG_STATUS_FILEINVALID.

(closes issue #15786)
 Reported by: a_villacis
 Patches: 
       asterisk-1.6.2.0-beta4-manager-fix-crash-on-include-nonexistent-file.patch uploaded by a villacis (license 660)
       (Plus a few of my own, to catch the remaining places within manager.c where it could have been a problem)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@214514 65c4cc65-6c06-0410-ace0-fbb531ad65f3