vmpage requires struct pmap to exist and contain a pm_stats field. As of
r308817, either AIM or BOOKE is required to be set in order to get their
respective pmap structs. Rather than expose them both, or try to unify them
unnecessarily, add a third option which contains only a pm_stats field, and
change the two existing pmap structures to place the common fields at the
beginning of the struct. This actually fixes the stats collection by libkvm on
AIM hardware, because before it was accessing a possibly different offset, which
would cause it to read garbage.
Bump __FreeBSD_version to denote this ABI change, so that ports which depend on
libkvm can be rebuilt.
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