Implement new ISA interface functions:
* isa_intr_alloc() - allocate an ISA IRQ
* isa_mem_alloc() - allocate ISA memory space
* isa_mem_free() - free ISA memory space
isa_intr_{,dis}establish()) here. Make this file safely includable by
<dev/isa/isavar.h> and by all of the places in the i386 that are currently
including it, which means multiple inclusion protection and a few
otherwise-extraneous 'forward' structure declarations. isa_intr_establish()
and isa_intr_disestablish() now take as their first argument a
machine-dependent cookie of type isa_chipset_tag_t, which is also defined
here.
wrap the various uses of the chip (including the uses of the NVRAM on it)
around the new defintions, structures, and functions specified in the header.
thorpej