http: cork chunked responses for small savings

http: cork chunked responses for small savings

This only affects Linux users with MSG_MORE support.

We can avoid extra TCP overhead for sub-optimal chunk sizes
by using MSG_MORE even with chunk trailers under Linux.

This breaks real-time apps which require <= 200ms latency for
streaming small packets (e.g. implementing "tail -F"), but the
public-inbox WWW code does not (and will never) do such things.