sd-dhcp: don't randomly ref objects

sd-dhcp: don't randomly ref objects

In our API design, getter-functions don't ref objects. Calls like
foo_get_bar() will not ref 'bar'. We never do that and there is no real
reason to do it in single threaded APIs. If you need a ref-count, you
better take it yourself *BEFORE* doing anything else on the parent object
(as this might invalidate your pointer).

Right now, sd_dhcp?_get_lease() refs the lease it returns. A lot of
code-paths in systemd do not expect this and thus leak the lease
reference. Fix this by changing the API to not ref returned objects.