MEDIUM: mux-h2: discard contents that are to be sent after a shutdown

MEDIUM: mux-h2: discard contents that are to be sent after a shutdown

In h2_snd_buf() we discard any possible buffer contents requested to be
sent after a close or an error. But in practice we can extend this to
any case where the stream is locally half-closed since it means we will
never be able to send these data anymore.

For now it must not change anything, but it will be used by subsequent
patches to discard lone a HTX EOM block arriving after the trailers
block.