Keep track of digit duration, when we're decoding inband to pass DTMF frames.

(closes issue #17235)
 Reported by: frawd
 Patches: 
       new_dtmf_dsp_len.patch uploaded by frawd (license 610)
       20100518__issue17235.diff.txt uploaded by tilghman (license 14)
 Tested by: frawd


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@264204 65c4cc65-6c06-0410-ace0-fbb531ad65f3