res_pjsip_messaging: Register message technology as pjsip

pjsip's message technology was being registered as 'sip', which was causing it
to not load due it conflicting with chan_sip's registered 'sip' technology for
messaging.  It now registers as 'pjsip'.  However, due to this change the "to"
field for outgoing pjsip messages need to be prefixed with 'pjsip:' instead of
'sip:'.  Incoming messages to res_pjsip_messaging will automatically have their
"to" fields altered in order to accommodate the change.  Outgoing messages also
handle changing it back to 'sip' before being sent so the pjsip library will
properly handle it.

(closes issue ASTERISK-22445)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2833/
........

Merged revisions 399339 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399340 65c4cc65-6c06-0410-ace0-fbb531ad65f3