* fhandler.h (fhandler_serial::fhandler_serial): Change to only accept unit

* fhandler.h (fhandler_serial::fhandler_serial): Change to only accept unit
argument.
* fhandler_serial.cc (fhandler_serial::fhandler_serial): Ditto.
(fhandler_serial::open): Avoid else when previous clause is a return().
* path.cc (get_devn): Alias /dev/ttyS0 -> /dev/com1, etc.
(get_device_number): Reallow standalone "com1" as a valid name for /dev/com1.