Boats: Properly use 'dtime' for consistent responsiveness

Previously, boat responsiveness varied greatly depending on server
lag and singleplayer / multiplayer.
'dtime' is the time from the last execution of the boat entity
'on step' function, so must multiply acceleration and yaw change.

Retune acceleration, turn rate and drift deceleration.

Make reversed turn only happen with significant backwards speed.

mods/boats/init.lua

@@ -141,26 +141,26 @@ function boat.on_step(self, dtime)
 					"[boats] Cruise on")
 			end
 		elseif ctrl.down then
-			self.v = self.v - 0.1
+			self.v = self.v - dtime * 1.8
 			if self.auto then
 				self.auto = false
 				minetest.chat_send_player(driver_name,
 					"[boats] Cruise off")
 			end
 		elseif ctrl.up or self.auto then
-			self.v = self.v + 0.1
+			self.v = self.v + dtime * 1.8
 		end
 		if ctrl.left then
-			if self.v < 0 then
-				self.object:set_yaw(self.object:get_yaw() - (1 + dtime) * 0.03)
+			if self.v < -0.001 then
+				self.object:set_yaw(self.object:get_yaw() - dtime * 0.9)
 			else
-				self.object:set_yaw(self.object:get_yaw() + (1 + dtime) * 0.03)
+				self.object:set_yaw(self.object:get_yaw() + dtime * 0.9)
 			end
 		elseif ctrl.right then
-			if self.v < 0 then
-				self.object:set_yaw(self.object:get_yaw() + (1 + dtime) * 0.03)
+			if self.v < -0.001 then
+				self.object:set_yaw(self.object:get_yaw() + dtime * 0.9)
 			else
-				self.object:set_yaw(self.object:get_yaw() - (1 + dtime) * 0.03)
+				self.object:set_yaw(self.object:get_yaw() - dtime * 0.9)
 			end
 		end
 	end
@@ -170,7 +170,7 @@ function boat.on_step(self, dtime)
 		return
 	end
 	local s = get_sign(self.v)
-	self.v = self.v - 0.02 * s
+	self.v = self.v - dtime * 0.6 * s
 	if s ~= get_sign(self.v) then
 		self.object:set_velocity({x = 0, y = 0, z = 0})
 		self.v = 0