resolved: rework how we allow allow queries to be dispatched to scopes

resolved: rework how we allow allow queries to be dispatched to scopes

Previously, we'd never do any single-label or root domain lookups via
DNS, thus leaving single-label lookups to LLMNR and the search path
logic in order that single-label names don't leak too easily onto the
internet. With this change we open things up a bit, and only prohibit
A/AAAA lookups of single-label/root domains, but allow all other
lookups. This should provide similar protection, but allow us to resolve
DNSKEY+DS RRs for the top-level and root domains.

(This also simplifies handling of the search domain detection, and gets
rid of dns_scope_has_search_domains() in favour of
dns_scope_get_search_domains()).