Restoring some code to sig_pri. Not sure if it is really needed.

Putting some DSP code back into sig_pri that was removed by the
chan_dahdi/sig_pri reorganization.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@211392 65c4cc65-6c06-0410-ace0-fbb531ad65f3