I found that for a package marked as on hold,
doing forbid-version
then unhold
also wipes out forbid-version.

Sure, usually this is what one wants, but this side effect is not
mentioned on the man page, not under forbid-version, not under unhold.

I.e., if we want to change from hold, to just forbid-version, well,
there will have to be a moment when we completely unmark the package...