Add same speedup from Arduino core, that on Serial1.write, check to see
if output queue is empty and data register empty, if so simply stuff the
character out in the output queue, without needing to use interrupt to
start transmission.  Helps speed up fast IO like 1MBS

Also, changed how UCSR1B is updated, to be like Arduino.  In particular
it turns on and off specific bit(s) instead of complete set of register.
Needed if you wish to emulate half duplex with this uart.