resolved: make dns_transaction_gc return a pointer

resolved: make dns_transaction_gc return a pointer

_gc() does cleanup if it is possible. So far it returned a bool to
signal if it succeeded (false on success). When working on the resolved
code I had to look at the definition every time, because the (arguably
reversed) calling convention is unobvious. So let's return a pointer
(non-NULL: gc has not been done, NULL: gc has been done).

This fits nicely with the standard to return a pointer from all free
functions obviously.