Several changes to codec_dahdi to play nice with G723.

This commit brings in the changes that were living out on the
svn/asterisk/team/sruffell/asterisk-trunk-transcoder branch.  codec_dahdi.c now
always uses signed linear as the simple codec so that a soft g729 codec will
not end up being preferred to the hardware codec.  There are also changes to
allow codec_dahdi.c to feed packets to the hardware in the native sample size of
the codec.  This solves problems with choppy audio when using G723. 



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@176760 65c4cc65-6c06-0410-ace0-fbb531ad65f3