MEDIUM: h2: send DATA+ES or RST_STREAM on shutw/shutr

MEDIUM: h2: send DATA+ES or RST_STREAM on shutw/shutr

When a stream sends a shutw, we send an empty DATA frame with the ES
flag set, except if no HEADERS were sent, in which case we rather send
RST_STREAM. On shutr(1) to abort a request, an RST_STREAM frame is sent
if the stream is OPEN and the stream is closed. Care is taken to switch
the stream's state accordingly and to avoid sending an ES bit again or
another RST once already done.