MINOR: lua: Make set_var() and unset_var() return success

This patch makes `set_var()` and `unset_var()` return a boolean indicating
success.

reg-tests/lua/set_var.lua

@@ -0,0 +1,12 @@
+core.register_service("set_var", "http", function(applet)
+	local var_name = applet.headers["var"][0]
+	local result = applet:set_var(var_name, "value")
+	if result then
+		applet:set_status(202)
+	else
+		applet:set_status(400)
+	end
+	applet:add_header("echo", applet:get_var(var_name) or "(nil)")
+	applet:start_response()
+	applet:send("")
+end)