[patch] Vehicle steady speed at speed limit

[TurfIt]

Vehicles in Simutrans drive like my Aunt! Too fast, too slow, too fast...

This patch lets vehicles brake to the speed limit instead of over slowing.
Overspeed on downhills can still occur.

[Dwachs]

The patch is incorporated in trunk (rev 3919). I rearranged the if's somewhat.

The rest of the topic is more like an extension request -> split and moved:
http://forum.simutrans.com/index.php?topic=6150.0