Adding busy-level to the SIP_PEER() dialplan function.

With this, you can control the peer in the dialplan, so you avoid placing outbound
calls when the device has reached busy-level.
Reported by pj.

Closes bug #11180



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89406 65c4cc65-6c06-0410-ace0-fbb531ad65f3