since BTLB entries can be scarce and very little of an I/O subsystem normally
needs to be mapped.
Instead, the pmap now allows mappings of I/O space to be entered with
pmap_kenter_pa. bus_space mappings for small amounts of I/O space (as for
virtually all devices) are made this way, with BTLB entries still used for
large mappings for things like framebuffers.
This has led to more and cleaned-up uses of bus_space(9) and has caused
some autoconf cleanup. Also, kgdb is now attached and connected before
autoconfiguration, which is much earlier than before.
fredette