Allow installer updating only without updating files

In phpBB 3.1.x, there was possible to update database only using /ionstall/database_update.php using any phpBB package (full/autoupdate).
In 3.2.0-a1, this is not the case anymore, so it's impossible to update database only with the full package (or with updatre package if phpBB version to update from mismatch).

It should be possible to update database unconditionally with the new installer.