make ast_read() able to handle channel read()/exception() methods that return a chain of frames

cleanup code in ast_read()
add AST_FRAME_DTMF_BEGIN and AST_FRAME_DTMF_END so that variable-length DTMF events can be supported
teach chan_zap to send DTMF_BEGIN and DTMF_END when appropriate


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8850 65c4cc65-6c06-0410-ace0-fbb531ad65f3