Allow alternative extensions to be specified for a user.

(closes issue #12830)
 Reported by: jcollie
 Patches: 
       astertisk-trunk-121496-alternate-extensions.patch uploaded by jcollie (license 412)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@124049 65c4cc65-6c06-0410-ace0-fbb531ad65f3