Prevent conflicts regarding unsolicited and solicited MWI to an endpoint.

Prevent conflicts regarding unsolicited and solicited MWI to an endpoint.

If an endpoint is receiving unsolicited MWI for a mailbox and then attempts
to subscribe to an AOR that provides MWI for the same mailbox, then the SUBSCRIBE
is rejected with a 500 response.

Review: https://reviewboard.asterisk.org/r/3345
........

Merged revisions 410590 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410591 65c4cc65-6c06-0410-ace0-fbb531ad65f3