Boats: Add cruise mode (boat autoforward)

Document controls in README.txt.

Optimise 'get yaw' and 'set yaw' code.

mods/boats/README.txt

@@ -13,3 +13,19 @@ Textures: Zeg9 (CC BY-SA 3.0)
Model: thetoon and Zeg9 (CC BY-SA 3.0),
  modified by PavelS(SokolovPavel) (CC BY-SA 3.0),
  modified by sofar (CC BY-SA 3.0)

Controls
--------
Right mouse button = Enter or exit boat when pointing at boat.
Forward            = Speed up.
                     Slow down when moving backwards.
Forward + backward = Enable cruise mode: Boat will accelerate to maximum forward
                     speed and remain at that speed without needing to hold the
                     forward key.
Backward           = Slow down.
                     Speed up when moving backwards.
                     Disable cruise mode.
Left               = Turn to the left.
                     Turn to the right when moving backwards.
Right              = Turn to the right.
                     Turn to the left when moving backwards.

mods/boats/init.lua

@@ -44,7 +44,8 @@ local boat = {
	driver = nil,
	v = 0,
	last_v = 0,
	removed = false
	removed = false,
	auto = false
}


@@ -55,6 +56,7 @@ function boat.on_rightclick(self, clicker)
	local name = clicker:get_player_name()
	if self.driver and clicker == self.driver then
		self.driver = nil
		self.auto = false
		clicker:set_detach()
		player_api.player_attached[name] = false
		player_api.set_animation(clicker, "stand" , 30)
@@ -130,24 +132,35 @@ end
function boat.on_step(self, dtime)
	self.v = get_v(self.object:get_velocity()) * get_sign(self.v)
	if self.driver then
		local driver_name = self.driver:get_player_name()
		local ctrl = self.driver:get_player_control()
		local yaw = self.object:get_yaw()
		if ctrl.up then
			self.v = self.v + 0.1
		if ctrl.up and ctrl.down then
			if not self.auto then
				self.auto = true
				minetest.chat_send_player(driver_name,
					"[boats] Cruise on")
			end
		elseif ctrl.down then
			self.v = self.v - 0.1
			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
		end
		if ctrl.left then
			if self.v < 0 then
				self.object:set_yaw(yaw - (1 + dtime) * 0.03)
				self.object:set_yaw(self.object:get_yaw() - (1 + dtime) * 0.03)
			else
				self.object:set_yaw(yaw + (1 + dtime) * 0.03)
				self.object:set_yaw(self.object:get_yaw() + (1 + dtime) * 0.03)
			end
		elseif ctrl.right then
			if self.v < 0 then
				self.object:set_yaw(yaw + (1 + dtime) * 0.03)
				self.object:set_yaw(self.object:get_yaw() + (1 + dtime) * 0.03)
			else
				self.object:set_yaw(yaw - (1 + dtime) * 0.03)
				self.object:set_yaw(self.object:get_yaw() - (1 + dtime) * 0.03)
			end
		end
	end