Make ISA PnP work. This successfully enumerates as many ISA devices as I
can fit into my test machine.

• Move to using STAILQs rather than ad-hoc singly-linked lists.
• Use a mostly procedural interface to the PnP information. This improves data-hiding.

Implement a new linker-set technique (currently on i386 only but should work
on Alpha as well). This is a good candidate for replacing the current
gensetdefs cruft completely.