Add .luacheckrc and fix issues it pointed out (#589)

.luacheckrc

@@ -0,0 +1,36 @@
+std = "lua51c"
+
+ignore = {
+	"21/_+", -- Unused variable, except "_", "__", etc.
+	"213", -- Unused loop variable
+	"421", -- Shadowing a local variable
+	"422", -- Shadowing an argument
+	"423", -- Shadowing a loop variable
+	"431", -- Shadowing an upvalue
+	"432", -- Shadowing an upvalue argument
+	"433", -- Shadowing an upvalue loop variable
+	"542", -- Empty if branch
+}
+
+max_line_length = 200
+
+read_globals = {
+	"default",
+	"digiline",
+	"doors",
+	"dump",
+	"jit",
+	"minetest",
+	"screwdriver",
+	"string.split",
+	"table.copy",
+	"table.insert_all",
+	"vector",
+	"VoxelArea",
+}
+
+globals = {"mesecon"}
+
+files["mesecons/actionqueue.lua"] = {
+	globals = {"minetest.registered_globalsteps"},
+}