MINOR: mux-h2: clear up H2_CF_DEM_DFULL and H2_CF_DEM_SHORT_READ ambiguity

MINOR: mux-h2: clear up H2_CF_DEM_DFULL and H2_CF_DEM_SHORT_READ ambiguity

Since commit 485da0b05 ("BUG/MEDIUM: mux_h2: Handle others remaining
read0 cases on partial frames"), H2_CF_DEM_SHORT_READ is set when there
is no blocking flags. However, it checks H2_CF_DEM_BLOCK_ANY which does
not include H2_CF_DEM_DFULL. This results in many cases where both
H2_CF_DEM_DFULL and H2_CF_DEM_SHORT_READ are set together, which makes
no sense, since one says the demux buffer is full while the other one
says an incomplete read was done. This doesn't permit to properly
decide whether to restart reading or processing.

Let's make sure to clear DFULL in h2_process_demux() whenever we
consume incoming data from the dbuf, and check for DFULL before
setting SHORT_READ.

This could probably be considered as a bug fix but it's hard to say if
it has any impact on the current code, probably at worst it might cause
a few useless wakeups, so until there's any proof that it needs to be
backported, better not do it.