With IMAP backend, messages in INBOX were counted twice for MWI.

(closes issue #17135)
 Reported by: edhorton
 Patches: 
       20100513__issue17135.diff.txt uploaded by tilghman (license 14)
       17135_2.diff uploaded by ebroad (license 878)
 Tested by: edhorton, ebroad


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@263589 65c4cc65-6c06-0410-ace0-fbb531ad65f3