resolved: keep addresses mapped to ::0 in a separate set

resolved: keep addresses mapped to ::0 in a separate set

We'd store every 0.0.0.0 and ::0 entry as a structure without any addresses
allocated. This is a somewhat common use case, let's optimize it a bit.

This gives some memory savings and a bit faster response time too:
'time build/test-resolved-etc-hosts hosts' goes from 7.7s to 5.6s, and
memory use as reported by valgrind for ~10000 hosts is reduced
==18097==   total heap usage: 29,902 allocs, 29,902 frees, 2,136,437 bytes allocated
==18240==   total heap usage: 19,955 allocs, 19,955 frees, 1,556,021 bytes allocated

Also rename 'suppress' to 'found' (with reverse meaning). I think this makes
the intent clearer.