resolve: use exact-match domain as routing domain for single-labels

resolve: use exact-match domain as routing domain for single-labels

With ResolveUnicastSingleLabel=yes, a scope's search domains are affixed to the
query even when a routing domain matches the single-label query name,
preventing the use of dotless single-label domains entirely.

This changes dns_scope_name_wants_search_domain() to return false when the
scope contains an exact match of the single-label name in the query, allowing
lookups for dotless domains with ResolveUnicastSingleLabel enabled.