Slippery nodes

Updated and improved 'slippery' factor in a new groups setting(I'd
prefer to have a new property, but that'd require a new protocol, which
may not be wished?  Suggestions?).

Modified from: Zeg9's: minetest#817

src/localplayer.h

@@ -50,7 +50,7 @@ public:
 	void move(f32 dtime, Environment *env, f32 pos_max_d,
 			std::vector<CollisionInfo> *collision_info);
 
-	void applyControl(float dtime);
+	void applyControl(float dtime, Environment *env);
 
 	v3s16 getStandingNodePos();