res_pjsip: add CLI command to show global and system configuration

Added a new CLI command for res_pjsip that shows both global and system
configuration settings: pjsip show settings

ASTERISK-24918 #close
Reported by: Scott Griepentrog
Review: https://reviewboard.asterisk.org/r/4597/
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Merged revisions 434527 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434528 65c4cc65-6c06-0410-ace0-fbb531ad65f3