resolved: allow dns_cache_put() without a question

resolved: allow dns_cache_put() without a question

Currently, dns_cache_put() does a number of things:

1) It unconditionally removes all keys contained in the passed
   question before adding keys from the newly arrived answers.

2) It puts positive entries into the cache for all RRs contained
   in the answer.

3) It creates negative entries in the cache for all keys in the
   question that are not answered.

Allow passing q = NULL in the parameters and skip 1) and 3), so
we can use that function for mDNS responses. In this case, the
question is irrelevant, we are interested in all answers we got.