[PKT_SCHED]: GRED: Use new generic red interface

Simplifies code a lot by separating the red algorithm and the
queueing logic. We now differentiate between probability marks
and forced marks but sum them together again to not break
backwards compatibility.

This brings GRED back to the level of RED and improves the
accuracy of the averge queue length calculations when stab
suggests a zero shift.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>