-Refactor P_XYMovement for clarity.

git-svn-id: http://odamex.net/svn/root/trunk@4487 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Rename PIT_RadiusAttack to PIT_DoomRadiusAttack.
-Change PIT_DoomRadiusAttack to respect sv_splashfactor like PIT_ZDoomRadiusAttack.

git-svn-id: http://odamex.net/svn/root/trunk@4488 b0bcef46-2027-0410-bd56-ab726e9a1d04