BUG/MEDIUM: mux-h2: always process a pending shut read

BUG/MEDIUM: mux-h2: always process a pending shut read

During 2.4-dev, an issue with partial frames was fixed with commit
3d4631fec ("BUG/MEDIUM: mux-h2: fix read0 handling on partial frames").
However this patch is not completely correct. It makes h2_recv() return
0 if the connection was shut for reads, but this not make h2_io_cb()
call h2_process(), so if there are any pending data left in the demux
buffer, they will never be processed, and the I/O callback will be
called in loops forever from the poller.

The correct return value there is 1, as is done at the end of the
function to report a pending read0.

This should definitely fix issue #1328. However even after a lot of
tests I couldn't manage to reproduce it, the conditions to enter that
situation are quite racy.

This must be backported to 2.0 since the fix above was merged into
2.0.21 and 2.2.9.