The head of the list is now a pv_header, which contains the first pv_entry
as well as a 16-bit attributes field (replaces the pmap_attributes array
plus the pv_entry::pv_flags field) as a 16-bit count of caller-specified
cache-inhibited mappings.
Tested on hp300 (shared pmap_motorola.c), changes to atari and amiga copies
are purely mechanical.