Raspberry Pi driver:

- Correct check for mmap errors (closes #3604495).
- Add 40x4 support (for displays with two controllers).
- Update the documentation.

server/drivers/hd44780-rpi.h

@@ -11,6 +11,7 @@ int hd_init_rpi(Driver *drvthis);
  * stored here is used for mapping physical GPIO pins to BCM2835 gpio. */
 struct rpi_gpio_map {
 	int en;
+	int en2;
 	int rs;
 	int d7;
 	int d6;