Use a separate admission limit from the backpressure status threshold

A backpressure status threshold (informational, sets a status bit) and a hard grant gate are different jobs and may need different values. Reusing one parameter for both couples when the warning fires with when grants stop. Add a dedicated GRANT_LIMIT parameter and keep the status threshold unchanged, e.g. status = occ >= BP_LEVEL, available = occ >= GRANT_LIMIT ? 0 : GRANT_LIMIT - occ - reserved.