Added the 'a' option to app dial and modified app_dial to set the answertime when...

Added the 'a' option to app dial and modified app_dial to set the answertime when the called channel answers.

This change causes answertime to be correct even if the called channel hangs up during an announcement triggered by the A() option.

(closes issue #15936)
Reported by: falves11
Patches:
      dial-macro-billsec-fix1.diff uploaded by mnicholson (license 96)
      dial-caller-answer1.diff uploaded by mnicholson (license 96)
Tested by: falves11, mnicholson

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@227897 65c4cc65-6c06-0410-ace0-fbb531ad65f3