Incomplete list of things that this driver does not yet implement completely or
at all. some of these may not be possible to implement because the hardware
support does not exist. Others may be possible, but the magic control codes to
make them happen are unknown, and some may just need the driver support to
implement them writing.

* Mark/Space parity is not implemented (reported back correctly)
* IXANY flow control mode is not implemented (flag ignored completely)