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.
we want it (we don't support the 020 without an 851 anyway). Note that
M68030 no longer implies support for 020+851.
This also allows us to compile with -m68030 iff M68020 isn't specified
in the config file.
