Use event2str consisitently in chan_zap anytime we are printing debug info...

Use event2str consisitently in chan_zap anytime we are printing debug info about an event (bug 2548)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@3871 65c4cc65-6c06-0410-ace0-fbb531ad65f3