util.startup: Break out command line argument parsing into util.argparse

This will allow using it from other places such as prosodyctl
sub-commands and plugins

util/argparse.lua

@@ -0,0 +1,58 @@
+local function parse(arg, config)
+	local short_params = config and config.short_params or {};
+	local value_params = config and config.value_params or {};
+
+	local parsed_opts = {};
+
+	if #arg == 0 then
+		return parsed_opts;
+	end
+	while true do
+		local raw_param = arg[1];
+		if not raw_param then
+			break;
+		end
+
+		local prefix = raw_param:match("^%-%-?");
+		if not prefix then
+			break;
+		elseif prefix == "--" and raw_param == "--" then
+			table.remove(arg, 1);
+			break;
+		end
+		local param = table.remove(arg, 1):sub(#prefix+1);
+		if #param == 1 and short_params then
+			param = short_params[param];
+		end
+
+		if not param then
+			print("Unknown command-line option: "..tostring(param));
+			print("Perhaps you meant to use prosodyctl instead?");
+			os.exit(1);
+		end
+
+		local param_k, param_v;
+		if value_params[param] then
+			param_k, param_v = param, table.remove(arg, 1);
+			if not param_v then
+				print("Expected a value to follow command-line option: "..raw_param);
+				os.exit(1);
+			end
+		else
+			param_k, param_v = param:match("^([^=]+)=(.+)$");
+			if not param_k then
+				if param:match("^no%-") then
+					param_k, param_v = param:sub(4), false;
+				else
+					param_k, param_v = param, true;
+				end
+			end
+		end
+		parsed_opts[param_k] = param_v;
+	end
+	return parsed_opts;
+end
+
+return {
+	parse = parse;
+}