Preserve DTMF length in main/features.c

Review: https://reviewboard.asterisk.org/r/1463/

A small part of much larger work with DTMF duration in Asterisk, 
funded  by IPvision AS in Denmark.

Thanks to irroot for the review!



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@338623 65c4cc65-6c06-0410-ace0-fbb531ad65f3