sd-bus: add sd_bus_message.verify_destination_id and .destination_ptr

sd-bus: add sd_bus_message.verify_destination_id and .destination_ptr

kdbus learned to accept both a numerical destination ID as well as a
well-known-name. In that case, kdbus makes sure that the numerical ID is in
fact the owner of the provided name and fails otherwise.

This allows for race-free assertion of a bus name owner while sending a
message, which is a requirement for bus-proxyd.

Add two new fields to sd_bus_message, and set the numerical ID to
verify_destination_id if bus_message_setup_kmsg() is called for a
message with a well-known name.

Also, set the destination's name in the kdbus item to .destination_ptr
if it is non-NULL.

Normal users should not touch these fields, and they're not publicy
accessible.