SBUS: Add an async request to retrieve the caller ID
    
    Adds an async request sbus_get_sender_id_{send,recv} that allows
    retrieval of UID based on "sender" as returned by
    dbus_message_get_sender().
    
    The UID is an int64_t to be able to use "-1" to as a fallback value for
    uknown or error cases.
    
    The unit test is added as a standalone one, not part of the sbus_tests
    because the request, and by extension the unit test relies on being
    connected to the system bus, which is very unlikely to work in a build
    system.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Stef Walter <stefw@redhat.com>
    (cherry picked from commit 0161a3c5637a0c0092bf54c436bb3d6508d7df26)
    
    Conflicts:
    	Makefile.am