red: harddrop support and cleanups

Add harddrop support (kernel support added a long time ago), and various
cleanups.

min BYTES, max BYTES are now optional and follow Sally Floyd's
recommendations.

By the way, our default 2% probability is a bit low, Sally recommends 10%.
Not a big deal if upcoming adaptative algo is deployed.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
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