2.4.x backport of neighbour cache rework

I've finished my 2.4.28 and 2.4.21 backports of our recent neighbour cache re-work (see netdev of last two weeks in case you're interested). 2.4.28 was quite straight-forward, just the missing per-CPU hurt a bit. 2.4.21 was pretty hard, since the neighbour cache apparently changed quite a bit between 2.4.21 and 2.4.28.

But well, it's over now. Thank god :)