voicemail: Simplify callback pointer declarations and add doxygen.

* Typedefed and added doxegen for the voicemail callback functions.

* Simplified the prototypes for ast_install_vm_functions() and
ast_install_vm_test_functions() to use the new function typedefs.

* Simplified the voicemail callback function pointer variable declarations
to use the new function typedefs.
........

Merged revisions 402398 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402406 65c4cc65-6c06-0410-ace0-fbb531ad65f3