networkd previously knew an enum "ResolveSupport" for configuring
per-interface LLMNR support, resolved had a similar enum just called
"Support", with the same value and similar pasers.
Unify this, call the enum ResolveSupport, and port both daemons to it.
The constraints we place on the pool is that it is a contiguous
sequence of addresses in the same subnet as the server address, not
including the subnet nor broadcast addresses, but possibly including
the server address itself. If the server address is included in the
pool it is (obviously) reserved and not handed out to clients.