Add support for monitoring MWI on FXO lines.

Add support for monitoring MWI on FXO lines.

This introduces two new options for zapata.conf: mwimonitor and mwimonitornotify.
The mwimonitor option enables MWI monitoring.  When the MWI state on a line changes,
then the script specified by mwimonitornotify will be executed for custom handling
of the state change, similar to the externnotify option of voicemail.conf.

Also, when the MWI state on an FXO line changes, an internal Asterisk event is
generated to indicate the new state of the associated mailbox.  That may, any
module that cares about MWI information will get notified and can handle it
just as if app_voicemail had sent this notification.

(BE-253, original patch from markster, with some minor modifications by me to
 add comments, documentation, and internal event support)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@90949 65c4cc65-6c06-0410-ace0-fbb531ad65f3