Check if polarityonanswerdelay has elapsed before setting a channel as answered

after a polarity reversal.

Previously on a polarity switch event chan_dahdi would set the channel
immediately as answered. This would cause problems if a polarity reversal
occurred when the line was picked up as the dial would not have yet occurred. 
Now if the polarity reversal occurs before delay has elapsed after coming off
hook or an answer, it is ignored. Also, some refactoring was done in
_handle_event.

(closes issue #13917)
Reported by: alecdavis
Patches:
      chan_dahdi.bug13917.feb09.diff2.txt uploaded by alecdavis (license 585)
Tested by: alecdavis


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@203672 65c4cc65-6c06-0410-ace0-fbb531ad65f3