res_pjsip: Add PJSIP CLI commands

Implements the following cli commands:
pjsip list aors
pjsip list auths
pjsip list channels
pjsip list contacts
pjsip list endpoints
pjsip show aor(s)
pjsip show auth(s)
pjsip show channels
pjsip show endpoint(s)

Also...
Minor modifications made to the AMI command implementations to facilitate
reuse.

New function ast_variable_list_sort added to config.c and config.h to implement
variable list sorting.

(issue ASTERISK-22610)
patches:
  pjsip_cli_v2.patch uploaded by george.joseph (License 6322)
........

Merged revisions 404480 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404507 65c4cc65-6c06-0410-ace0-fbb531ad65f3