app_record: Add an option that allows DTMF '0' to act as an additional terminator

Using this terminator when active results in ${RECORD_STATUS} being set to
'OPERATOR' instead of 'DTMF'

(closes issue AFS-7)

Review: https://reviewboard.asterisk.org/r/3041/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403414 65c4cc65-6c06-0410-ace0-fbb531ad65f3