resolved: never allow _gateway lookups to go to the network

Make them rather fail than go to the network.

Previously we'd filter them on LLMNR (explicitly) and MDNS (implicitly,
because it doesn't have .local suffix), but not on DNS.

In order to make _gateway truly reliable, let's not allow it to go to
DNS either, and keep it local.

This is particular relevant, as clients can now request lookups without
local RR synthesis, where we'd rather have NXDOMAIN returned for
_gateway than have it hit the network.

(cherry picked from commit fbbc7218)