Make CEL behavior conform to the documentation

This modifies the behavior of the CEL engine to conform to documented
behavior for Asterisk 12 as defined on the wiki
https://wiki.asterisk.org/wiki/display/AST/Asterisk+12+CEL+Specification

The primary changes deal with removal of the peer field from function
calls since it is no longer directly relevant to the bridging system
and removal of the layer of CDR-like business logic that was providing
a partial emulation of Asterisk 11 CEL functionality. With this change,
there is no longer a distinction between "bridges" and "conferences"
and all participation changes are denoted with bridge enter and bridge
exit messages.

This updates the CEL unit tests to handle these changes and simplifies
some of the macros used in the process.

This also fixes a segfault when attempting to ref a configuration that
failed to load.

Review: https://reviewboard.asterisk.org/r/2788/
(issue ASTERISK-21567)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397431 65c4cc65-6c06-0410-ace0-fbb531ad65f3