vmtotal: extend memory counters to accomodate for current and future
hardware sizes.

32bit counters already overflow on approachable virtual memory page
counts, and soon would overflow on the physical pages counts as well.
Bump sizes to 64bit types. Bump __FreeBSD_version.

It is impossible to provide perfect backward ABI compat for this
change. If a program requests an old structure, it can be detected by
size. But if it queries the size first by passing NULL old req
pointer, there is almost nothing we can do to detect the desired ABI.
As a partial solution, check p_osrel of the quering process when
selecting the size to report.

Submitted by: Pawel Biernacki <pawel.biernacki@gmail.com>
Differential revision: https://reviews.freebsd.org/D13018