improved test for DMA controller. A Sharp laptop I own (1996-ish) seems
to have a fresh-boot state where all 4 DMA channels read back
count=0xFFFF and base=0xFFFF which previously caused the program to
think there was no DMA controller.
sndsb: show what the DMA 8237 code found. Also, lack of a DMA controller
is no longer a reason not to run.
add code to check playback against whether or not DMA controllers are
present.