Unregister CLI commands on exit

This patch ensures that CLI commands enabled by DEBUG_FD_LEAKS and
DEBUG_THREADLOCALS are cleaned up properly on exit.

(closes issue ASTERISK-22238)
Reported by: Corey Farrell
Tested by: Corey Farrell
Patches:
    debug_cli_unregister.patch uploaded by Corey Farrell
........

Merged revisions 397106 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@397107 65c4cc65-6c06-0410-ace0-fbb531ad65f3