Make DTMF attended transfer support feature-complete.

This greatly modifies the operation of DTMF attended transfers so that
the full range of options from features.conf applies.

In addition, a new option has been added that allows for a transferer
to switch between bridges during a transfer before completing the
transfer.

(closes issue ASTERISK-21543)
reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/2654



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@395151 65c4cc65-6c06-0410-ace0-fbb531ad65f3