PJSIP: Restore functional default for callerid_privacy

The pjsip config option default fixups from r424263 altered the
functional default from "allowed_not_screened" to "allowed". This
change restores the functional default value when none is provided.
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Merged revisions 424426 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424427 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424428 65c4cc65-6c06-0410-ace0-fbb531ad65f3