Install BUILDINFO if available.

Install BUILDINFO if available.

We already keep /var/db/installed.SHA256 for sysupgrade(8) as a cheap
check to not install the same snapshot over and over again. With
/mnt/var/db/installed.BUILDINFO we can ensure that we are not
installing an older snapshot over a newer snapshot.

Intentionally not yet committing the sysupgrade(8) bits so that we
exercise the sysupgrade(8) / install.sub interaction as it will happen
during the 7.5 -> 7.6 transition. I.e. a newer install.sub getting
installed by an older sysupgrade(8).

deraadt@ likes it.