Add support for new PING/PONG on zaptel which reduces the # of file...

Add support for new PING/PONG on zaptel which reduces the # of file descriptors on SIP/IAX channels (when zaptel interface is available) by 2 per channel


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@2467 65c4cc65-6c06-0410-ace0-fbb531ad65f3