Add CLI "pri show channels" command.

List the current mapping of DAHDI B channels to Asterisk channel names and
which calls are on hold or call-waiting.  Calls on hold or call-waiting
are not associated with any B channel.

JIRA LIBPRI-27
JIRA SWP-2547


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@307964 65c4cc65-6c06-0410-ace0-fbb531ad65f3