MINOR: mux_quic: reset stream after app shutdown for HTTP/0.9

MINOR: mux_quic: reset stream after app shutdown for HTTP/0.9

HTTP/3 implements a GOAWAY frame for graceful shutdown. This allows to
reject any new stream opening with a larger ID. This is implemented via
HTTP/3 attach() callback called by qcs_new().

When HTTP/0.9 is used, there is no similar mechanism. This renders some
feature such as server max-reuse difficult to implement. This patch now
provides a method for such protocols with no graceful shutdown support.
Instead of invoking attach() callback, a stream is now immediately
resetted if the application protocol layer is already closed.

This patch does not change the behavior for HTTP/3. Only limited
protocols (currently only HTTP/0.9) without graceful shutdown are
impacted. These protocols are identified as their shutdown() callback is
nul.

This change is only necessary for HTTP/0.9 as there is no equivalent of
HTTP/3 GOAWAY in this case.