- if it wasn't there, just register it
- if it was there, don't allow it to go backwards
- when @version increases, anything goes. The new plist might have some
new objects, we don't really care.
as requested by sthen@
makes it possible to avoid REVISION bumps for large system changes (such
as gcc 4.9 -> 8.*)
it is advised to do a bulk with the newer register-plist first so that the
lists will have @version markers