BUG/MINOR: mux-h2: make sure to honor KILL_CONN in do_shut{r,w}

BUG/MINOR: mux-h2: make sure to honor KILL_CONN in do_shut{r,w}

If the stream closes and quits while there's no room in the mux buffer
to send an RST frame, next time it is attempted it will not lead to
the connection being closed because the conn_stream will have been
released and the KILL_CONN flag with it as well.

This patch reserves a new H2_SF_KILL_CONN flag that is copied from
the CS when calling shut{r,w} so that the stream remains autonomous
on this even when the conn_stream leaves.

This should ideally be backported to 1.9 though it depends on several
previous patches that may or may not be suitable for backporting. The
severity is very low so there's no need to insist in case of trouble.