Amateur radio uses 22ms bit timings for RTTY. This translates
to a baudrate of 45.45 repeating, so FreeBSD is not a good choice for
hardware-keyed FSK. Further, the Oxford PCIe driver defaults to a
clock rate that is too high to support 45 baud at all.
This allows notifying puc(4) of an upcoming baudrate so it can adjust
the prescaling to allow it. The Oxford drivers will now set up the
prescaler and oversampling to allow the closest possible solution for
a baud rate. This is currently only used by the new ioctl. Also, the
clock rate for the Oxford PCIe parts is 62.5MHz, it is not a multiple
of 1.8432MHz, so this also makes timing better for these devices.