This enables optional support for the hardware UART to use the RTS
and/or CTS pins for flow control.

The new "flow" constructor keyword specifies a bitmask of RTS and CTS.

Previously it was possible to specify which pins to use for the RTS and
CTS signals, but hardware flow control was never functional: CTS was not
checked before transmitting bytes, and RTS was always driven high
(signalling no buffer space available). With this patch, CTS and RTS
both operate as expected.

Signed-off-by: Will Sowerbutts will@sowerbutts.com