MEDIUM: mux-quic: implement shutw

MEDIUM: mux-quic: implement shutw

Implement mux_ops shutw operation for QUIC mux. A RESET_STREAM is
emitted unless the stream is already closed due to all data or
RESET_STREAM already transmitted.

This operation is notably useful when upper stream layer wants to close
the connection early due to an error.

This was tested by using a HTTP server which listens with PROXY protocol
support. The corresponding server line on haproxy configuration
deliberately not specify send-proxy. This causes the server to close
abruptly the connection. Without this patch, nothing was done on the QUIC
stream which was kept open until the whole connection is closed. Now, a
proper RESET_STREAM is emitted to report the error.

This should be backported up to 2.7.