Add ETSI Call Waiting support.

Add the ability to announce a call to an endpoint when there are no B
channels available.  A call waiting call is a SETUP message with no B
channel selected.

Relevant specification: EN 300 056, EN 300 057, EN 300 058

For DAHDI/ISDN channels, the CHANNEL() dialplan function now supports the
"no_media_path" option.
* Returns "0" if there is a B channel associated with the call.
* Returns "1" if no B channel is associated with the call.  The call is
either on hold or is a call waiting call.

If you are going to allow incoming call waiting calls then you need to use
CHANNEL(no_media_path) do determine if you must drop a call to accept the
new call.

Review:	https://reviewboard.asterisk.org/r/568/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@267261 65c4cc65-6c06-0410-ace0-fbb531ad65f3