cleaned up all standing in a sloped corner and getting hurt glitches by adding an...

cleaned up all standing in a sloped corner and getting hurt glitches by adding an SV_SetOnGround function which is called in SV_WalkMove in the right places (incase SV_FlyMove does not set it), which also fixed stair stepup smoothing (it needs FL_ONGROUND to not be flipping on and off during a step up)
added stair stepdown (instead of going flying off stairs)
added stair stepdown smoothing

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4373 d7cf8633-e32d-0410-b094-e92efae38249