BUG/MEDIUM: stconn: Block zero-copy forwarding if EOS/ERROR on consumer side

BUG/MEDIUM: stconn: Block zero-copy forwarding if EOS/ERROR on consumer side

When the producer side (h1 for now) negociates with the consumer side to
perform a zero-copy forwarding, we now consider the consumer side as blocked
if it is closed and this was reported to the SE via a end-of-stream or a
(pending) error.

It is performed before calling ->nego_ff callback function, in se_nego_ff().
This way, all consumer are concerned automatically. The aim of this patch is
to fix an issue with the QUIC mux. Indeed, it is unexpected to send a frame
on an closed stream. This triggers a BUG_ON(). Other muxes are not affected
but it remains useless to try to send data if the stream is closed.

This patch should fix the issue #2372. It must be backported to 2.9.