Allow a password change to be validated by an external script.

(closes issue #12090)
 Reported by: jaroth
 Patches: 
       vm-check-newpassword.diff.txt uploaded by mvanbaak (license 7)
       20080509__bug12090.diff.txt uploaded by Corydon76 (license 14)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@115582 65c4cc65-6c06-0410-ace0-fbb531ad65f3