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'.
The cache lookup was performed before rule matching, using the caller's
strategy (usually AsIS/0) instead of the resolved strategy. This caused
cache misses when ipv4_only was configured globally but the cache lookup
expected both A and AAAA records.
Remove LookupCache and ExchangeCache from Router, as the cache checks
inside client.Lookup and client.Exchange already handle caching correctly
after rule matching with the proper strategy and transport.
