Set up interrupt-level input buffering dynamically depending on the input speed, so that it can work at speeds larger than 115200 bps without being flow controlled.  The buffer is twice as large as before at 115200 bps and half as large as before at low speeds Use a single interrupt-level buffer instead of ping-pong buffers because the simplifications provided by ping-pong buffers became complications.
Set up interrupt-level input buffering dynamically depending on
the input speed, so that it can work at speeds larger than 115200
bps without being flow controlled.  The buffer is twice as large
as before at 115200 bps and half as large as before at low speeds
Use a single interrupt-level buffer instead of ping-pong buffers
because the simplifications provided by ping-pong buffers became
complications.

This change is over-engineered.  Statically configured buffering
was simpler and faster, and increasing the buffer size to support
1.5Mbps would cost about 1 US cent's worth of RAM per port, but I
was interested in the buffer switching mechanism.

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