voicemail API callbacks: Extract the sayname API call to its own registerd callback.

* Extract the sayname API call to its own registerd callback.  This allows
the app_directory and app_chanspy applications to say a mailbox owner's
name using an alternate provider when app_voicemail is not available
because you are using res_mwi_external.  app_directory still uses the
voicemail.conf file.

AFS-64 #close
Reported by: Mark Michelson


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@416830 65c4cc65-6c06-0410-ace0-fbb531ad65f3