mod_voicemail: Implement 10 new standard api function call that allow you to…

mod_voicemail: Implement 10 new standard api function call that allow you to control fs voicemail storage system.  The goal is to have a standard API set for any additional storage system we wish the voicemail to run off.  Current list of added api name are : vm_fsdb_msg_count, vm_fsdb_msg_list, vm_fsdb_msg_get, vm_fsdb_msg_delete, vm_fsdb_msg_undelete, vm_fsdb_msg_purge, vm_fsdb_msg_save, vm_fsdb_pref_greeting_set, vm_fsdb_pref_recname_set, vm_fsdb_pref_password_set.