Deterministically wait on hotplug_libusb shutdown

Change HPStopHotPluggables() to actually wait until the background
hotplug thread exits.

This makes sure that the hotplug mechanism doesn't continue working
throughout the daemon's shutdown process, and hence use-after-frees
(if a reader is added/removed after the readerfactory is shut down),
memory leaks etc. are prevented.

Note: this is expected to make the "SYS_Sleep(1)" trick in pcscdaemon.c
unnecessary, replacing it with a more reliable alternative. We don't
delete the sleep in the same commit in case it turns out to be crucial
for some other reason.

src/hotplug_libusb.c

@@ -576,7 +576,7 @@ LONG HPSearchHotPluggables(const char * hpDirPath)
 			return -1;
 		}
 
-		ThreadCreate(&usbNotifyThread, THREAD_ATTR_DETACHED,
+		ThreadCreate(&usbNotifyThread, 0,
 			(PCSCLITE_THREAD_FUNCTION( )) HPEstablishUSBNotifications, pipefd);
 
 		/* Wait for initial readers to setup */
@@ -596,6 +596,8 @@ LONG HPSearchHotPluggables(const char * hpDirPath)
 
 LONG HPStopHotPluggables(void)
 {
+	/* tell the rescan thread to shut down; it checks the ara kiri flag, but it
+	 * might also need to be awaken from reading the rescan pipe */
 	AraKiriHotPlug = true;
 	HPReCheckSerialReaders();
 
@@ -604,6 +606,8 @@ LONG HPStopHotPluggables(void)
 		close(rescan_pipe[1]);
 		rescan_pipe[1] = -1;
 	}
+	/* wait for the rescan thread to complete its cleanup */
+	pthread_join(usbNotifyThread, NULL);
 
 	return 0;
 }