BUG/MEDIUM: mux-h2: Report RST/error to app-layer stream during 0-copy fwding

BUG/MEDIUM: mux-h2: Report RST/error to app-layer stream during 0-copy fwding

In h2_nego_ff(), it is important to report reset and error to app-layer
stream and to send the RST-STREAM frame accordingly. It is not clear if it
is an issue or not. But it is clearly a difference with the classical
forwarding via h2_snd_buf. And it is mandatory for the next fix.

This patch should be backported to 3.2. But is is probably a good idea to
not backport it on older versions, except if a bug is reported in this area.