resolved: correctly handle non-address RR types with /etc/hosts lookups (#4808)

resolved: correctly handle non-address RR types with /etc/hosts lookups (#4808)

Fix wrong condition test in manager_etc_hosts_lookup(), which caused it to
return an IPv4 answer when an IPv6 question was asked, and vice versa.
Also only return success if we actually found any A or AAAA record.

In systemd-resolved.service(8), point out that /etc/hosts mappings only
affect address-type lookups, not other types.

The test case currently disables DNSSEC in resolved, as there is a bug
where "-t MX" fails due to "DNSSEC validation failed" even after
"downgrading to non-DNSSEC mode". This should be dropped once that bug
gets fixed.

Fixes #4801