DHCP option 119/15 and resolved link domains are not rooted, so appending
them to the query name produced a non-FQDN name, failing to pack with
"domain must be fully qualified".
Return SUCCESS with empty answers instead of an error when the
queried address family has no range configured. Reject configurations
where neither inet4_range nor inet6_range is set.
We mistakenly believed that `libresolv`'s `search` function worked correctly in NetworkExtension, but it seems only `getaddrinfo` does.
This commit changes the behavior of the `local` DNS server in NetworkExtension to prefer DHCP, falling back to `getaddrinfo` if DHCP servers are unavailable.
It's worth noting that `prefer_go` does not disable DHCP since it respects Dial Fields, but `getaddrinfo` does the opposite. The new behavior only applies to NetworkExtension, not to all scenarios (primarily command-line binaries) as it did previously.
In addition, this commit also improves the DHCP DNS server to use the same robust query logic as `local`.
Previously, the buffer was not reset within the response loop. If a packet
handle failed or completed, the buffer retained its state. Specifically,
if `ReadPacketFrom` returned `io.ErrShortBuffer`, the error was ignored
via `continue`, but the buffer remained full. This caused the next
read attempt to immediately fail with the same error, creating a tight
busy-wait loop that consumed 100% CPU.
Validates `buffer.Reset()` is called at the start of each iteration to
ensure a clean state for 'ReadPacketFrom'.
