dns-domain: tweak hash table comparison function for DNS names

dns-domain: tweak hash table comparison function for DNS names

Currently, when comparing two DNS names when storing them in a
hashtable, and the DNS names are not actually valid we'll compare the
error codes.

This is not very smart however, since this means two invalid DNS names
that happen to be equally "invalid" will be considered identical, even
if their strings are entirely different.

Let's find a better solution for this niche case: let's simple compare
the domains as strings.

This matters in case of DNS label compression: if we already added added
an invalid DNS name into the label compression hash table, and lookup
any other invalid DNS name, this lookup will likely return what the
earlier one already returned, and that's confusing.