rate control rework

This patchset is the result of my recent joint-venture with Mattias. It introduces a new rate control algorithm based on a PID controller. First tests show it works much better than the old simple rate control algorithm.

I think that this should really go into 2.6.25, as the current rate control algorithm was just written for reference and performs very poorly.

Johannes' and John's concerns have been addressed here. This patchset won't (hopefully!) get line-wrapped.

Patches 1 to 3 introduce the PID controller algorithm. Patches 4 and 5 are algorithm enhancements, while 6 to 8 are meant for debugging and tuning.

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Ciao
Stefano