Add 'stun show status' command

This patch adds a new CLI command, 'stun show status'.  This command will show
a table describing all known STUN servers and statuses.

(closes issue ASTERISK-18046)
Reported by: Jeremy Kister
Tested by: Jeremy Kister
patches:
  (stun-show-status-v4-trunk.patch license #6232 uploaded by Jeremy Kister)

Review: https://reviewboard.asterisk.org/r/2001



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369681 65c4cc65-6c06-0410-ace0-fbb531ad65f3