Add support for receiving unsolicited MWI NOTIFY messages.

This change adds a configuration option to SIP peers, unsolicited_mailbox, which
configures a virtual mailbox to use for received new/old MWI information. This
virtual mailbox can then be used by any device supporting MWI.

(closes issue #13028)
Reported by: AsteriskRocks
Patches:
      bug_13028_chan_sip_external_mwi_20090707.patch uploaded by cmaj (license 830)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@226060 65c4cc65-6c06-0410-ace0-fbb531ad65f3