MINOR: h2/trace: report the frame type when known

MINOR: h2/trace: report the frame type when known

In state match error cases, we don't know what frame type was received
because we don't reach the frame parsers. Let's add the demuxed frame
type and flags in the trace when it's known. For this we make sure to
always reset h2c->dsi when switching back to FRAME_H. Only one location
was missing. The state transitions were not always clear (sometimes
reported before, sometimes after), these were clarified by being
reported only before switching.