Prevent shutdown running more than once

Exit immediately if shutdown has already been called.  This is due to
the possibility of it being called multiple times from combinations of
signals and fatal errors.

packages/master/master.js

@@ -477,11 +477,16 @@ async function saveUsers(databaseDirectory, file) {
 	await fs.outputFile(filePath, JSON.stringify(serialized, null, 4));
 }
 
+let shuttingDown = false;
 /**
  * Innitiate shutdown of master server
  */
 async function shutdown() {
-	logger.info("Shutting down");
+	if (shuttingDown) {
+		return;
+	}
+	shuttingDown = true;
+
 	let exitStartTime = Date.now();
 	try {
 		logger.info("Saving configs");

packages/slave/slave.js

@@ -1384,6 +1384,9 @@ class Slave extends libLink.Link {
 	 * Stops all instances and closes the connection
 	 */
 	async shutdown() {
+		if (this._shuttingDown) {
+			return;
+		}
 		this._shuttingDown = true;
 		this.connector.setTimeout(30);