Serial port speed

[Guy Harris]

>Under 4.0.3, termio(4) and <sys/ttydev.h> agree that 38400 is as high as
>you can go unless you get more clever with EXTA or EXTB than getty may be
>able to handle.

EXTA is another name for 19.2; EXTB is another name for 38.4.  The speed
field is 4 bits long; what you see is what you get, and there ain't any
more speeds supported by the asynchronous-mode CPU serial port driver, nor
by the ALM-2 driver (I think the ALM-1 driver is stuck - the board doesn't
even support 38.4).