boats: Improve physics by implementing drag and friction forces

Implement drag force according to the equation:
drag_force = drag_coefficient * speed ^2
Also add a small constant force to implement friction force.

.luacheckrc

@@ -12,7 +12,9 @@ read_globals = {
 	"Settings",
 	"unpack",
 	-- Silence errors about custom table methods.
-	table = { fields = { "copy", "indexof" } }
+	table = { fields = { "copy", "indexof" } },
+	-- Silence warnings about accessing undefined fields of global 'math'
+	math = { fields = { "sign" } }
 }
 
 -- Overwrites minetest.handle_node_drops