Cleanups to fix bugs in the VM count API functions.

- Urgent voicemails were not attached, because the attachment code looked in the wrong folder.
- Urgent voicemails were sometimes counted twice when displaying the count of new messages.
- Backends were inconsistent as to which voicemails each API counted.
- Unit tests added to verify behavior in the future.

(closes issue #15654)
 Reported by: tomo1657
 Patches: 
       20100225__issue15654.diff.txt uploaded by tilghman (license 14)
 Tested by: tilghman

(closes issue #16448)
 Reported by: hevad

Review: https://reviewboard.asterisk.org/r/525/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@249187 65c4cc65-6c06-0410-ace0-fbb531ad65f3