Outgoing PRI/BRI calls cannot do DTMF triggered transfers.

Outgoing PRI/BRI calls cannot do DTMF triggered transfers.

Outgoing PRI/BRI calls cannot do DTMF triggered transfers if a PROCEEDING
message is not received.  The debug output shows that the DTMF begin event
is seen, but the DTMF end event is missing.  When the DTMF begin happens,
the call is muted so we now have one way audio (until a DTMF end event is
somehow seen).

* Made set the proceeding flag when the PRI_EVENT_ANSWER event is
received.

* Made absorb the DTMF begin and DTMF end events if we are overlap dialing
and have not seen a PROCEEDING message.

* Added a debug message when absorbing a DTMF event.

JIRA SWP-2690
JIRA ABE-2697

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@298193 65c4cc65-6c06-0410-ace0-fbb531ad65f3