No automatic superspeed when swimming / climbing & aux1_descends is on

First of all, sorry I took so long to fix this. Since adding the new physics, people who use aux1_descends complained about superspeed happening automatically when climbing ladders or swimming. That's because the 'fast' button is used as 'go down', so the two functions can no longer be separated into two separate keys.

I came to the conclusion that the best choice is to disable fast on ladders and underwater when aux1_descends is being used. It's sadly the only sane choice, and less annoying than always going fast.