Add %u and %g to the ASTERISK_PROMPT settings, for username and group,

respectively.  Also, take the opportunity to clean up the CLI prompt
generation code.
(closes issue #13175)
 Reported by: eliel
 Patches: 
       cliprompt.patch uploaded by eliel (license 64)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@134353 65c4cc65-6c06-0410-ace0-fbb531ad65f3