res_pjsip_outbound_registration: Add virtual line support.

Virtual line support establishes a relationship between messages
related to an outbound registration and a local endpoint. This is
accomplished by attaching a parameter to the Contact of the outbound
registration and looking for it on any received requests. If the
parameter exists and can be matched to an outbound registration
the configured endpoint is associated with the request.

Review: https://reviewboard.asterisk.org/r/2964/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@427165 f38db490-d61c-443f-a65b-d21fe96a405b