chan_pjsip: Add support for picking up calls in the configured pickup group.

AST-1363

Review: https://reviewboard.asterisk.org/r/3478/
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Merged revisions 413117 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@413118 65c4cc65-6c06-0410-ace0-fbb531ad65f3