Say: If SAY_DTMF_INTERRUPT is set to an ast_true value, jump on DTMF

Similar to how background works, if a say application is called with
this variable set to 'true', 'yes', 'on', etc. then using DTMF while
the say action is in progress will result in the channel jumping to
that extension in the dialplan.

Review: https://reviewboard.asterisk.org/r/3011/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402819 65c4cc65-6c06-0410-ace0-fbb531ad65f3