app_dial: Properly handle callee hangup while sending digits.

[InterLinked1]

If we are sending digits (either DTMF, MF, or SF) to the called channel
after receiving progress or a wink, and the callee hangs up before we
have finished sending it digits, there are several problems that can ensue:

• If the callee hung up without answering, the calling channel would
  hang up and not continue in the dialplan.
• If the callee did answer before hanging up, the answer was never
  passed through to the caller, since this gets "eaten" by the various
  digit streaming functions and is never processed by app_dial.

This is generally an edge case that occurs due to some kind of signaling
failure, but to better handle this:

• Set to_answer to 0 to prevent hangup on the exit path, just like other
  parts of wait_for_answer.
• Better document this usage of to_answer.
• If the channel did answer while it was receiving digits, manually
  answer the calling channel before we abort. The call would not continue
  in the dialplan anyways (either before or after this fix), but technically
  the call was answered, so the CDRs should probably reflect that, and this
  mirrors the behavior of calls which normally do not continue.

Resolves: #1915

[InterLinked1]

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