func_lock: Add "dialplan locks show" cli command.

For example:

arthur*CLI> dialplan locks show
func_lock locks:
Name                                     Requesters Owner
uls-autoref                              0          (unlocked)
1 total locks listed.

Obviously other potentially useful stats could be added (eg, how many
times there was contention, how many times it failed etc ... but that
would require keeping the stats and I'm not convinced that's worth the
effort.  This was useful to troubleshoot some other issues so submitting
it.

Change-Id: Ib875e56feb49d523300aec5f36c635ed74843a9f
Signed-off-by: Jaco Kroon <jaco@uls.co.za>