updated pkgdb instructions

["Thomas Mueller" <mueller6725%twc.com@localhost>]

From Greg Troxel:

> I have rototilled the instructions.  Keep in mind that nobody has yet
> tried them after the rototill, so tread carefully.

> https://pkgsrc.org/pkgdb-change/

> suggestions for clarification, pointing out errors, , positive and
> negative reports, etc, welcome.

What happens if user moves /usr/pkg, /usr/packages, /var/db/pkg and /var/db/pkg.refcount to backup locations?

I figure I need to start packages anew because of the current condition, also the desire to switch from modular xorg to native xorg.

Tom