- extdms_done is modified by things outside of our direct control;
it needs to be declared `volatile'
- CountADBs() fails when we call it the second time. We can work
around this by telling extdms_init() how many ADB devices we have,
only calling CountADBs() once.
Also change the device probing scheme to use something a bit more rational.
A current side-effect is that nubus cards are double-mapped. I expect
to fix that shortly.
Also change splclock() to block everything but serial hardware interrupts.
scottr