app_voicemail: Voicemail callback registration/unregistration function improvements.

* The voicemail registration/unregistration functions now take a struct of
callbacks instead of a lengthy parameter list of callbacks.

* The voicemail registration/unregistration functions now prevent a
competing module from interfering with an already registered callback
supplying module.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403643 65c4cc65-6c06-0410-ace0-fbb531ad65f3