Add CLI/AMI commands to force chan_pjsip actions

For chan_pjsip, this introduces CLI/AMI remote unregistration commands,
reworks CLI syntax for sending NOTIFYs, adds AMI qualification support,
and adds documentation for PJSIPNotify.

This also fixes two refcounting bugs in the outbound registration code.

Review: https://reviewboard.asterisk.org/r/2695/
(closes issue ASTERISK-21939)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@396087 f38db490-d61c-443f-a65b-d21fe96a405b