http: reswriter_flush: don't send end marker unless chunked

http: save the replied code in the reswriter struct

http: fix reswriter_write() return value we "wrote" only len(buf) bytes. Also, since res.dst is a dynstream no short writes can happen.

http: add illimitate buffering

http: fmt

http: introduce a reswriter; change API accordingly since it uses a bufio::stream it is limited to 1024 bytes of response, an auto-resizing implementation will follow...