Log Message:
pkg_install-20201218

- Support continuing to install to /var/db/pkg if it exists and the
new pkgdb doesn't.
In the future, we can warn about this once we have tested advice that
we can give to users who want to move the location of pkgdb.

- Don't do anything about /var/db/pkg on non-NetBSD-base.
This creates conflicts with other package managers that also install to
/var/db/pkg.