Properly return "free" on confirmed events that are free

CONFIRMED status doesn't imply busy or free, that is handled with the TRANSP
field. Luckily, libical already sets the is_busy status on the span for us.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@223370 65c4cc65-6c06-0410-ace0-fbb531ad65f3