1. Output sometimes gets stuck, i.e. when running "systat vm .1" on
the console. It seems that checking TXFE bit in the UARTFR
register in pluart_start() fixes that issue. But I restructured
that function a bit to be closer to comstart() and removed the
(unused) FIFO support code. I hope to add FIFO support later, but
for now it is just a distraction.
2. Tx interrupts aren't counted. To fix that I restructured the
interrupt handler a bit. It now actually looks at the interrupt