added select before write to avoid deadlock on full buffer. added some defines...

added select before write to avoid deadlock on full buffer. added some defines for deadlock debugging. added code snippet for generating silence if we don't have data to write.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@32524 65c4cc65-6c06-0410-ace0-fbb531ad65f3