Remove global symbol requirement from app_voicemail.

Remove global symbol requirement from app_voicemail.

This uses the existing "function installation" stuff that already existed for
other functions, like getting message counts.

(closes issue AST-807)
(issue AST-901)
(issue AST-908)

Review: https://reviewboard.asterisk.org/r/1965/
........

Merged revisions 368962 from http://svn.asterisk.org/svn/asterisk/certified/branches/1.8.11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10-digiumphones@368963 65c4cc65-6c06-0410-ace0-fbb531ad65f3