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  - When user #1 creates an account (we can assume this happens only once), system.module's schema version is set to the latest availiable.
  - system_get_files_database() now includes a 'schema_version' child of each file object.
  - That new information is re-saved when Drupal re-populates the system table.
  - An array of newly-enabled modules is built, module_list() is reloaded, and the schema versions of each newly-enabled module are set to the most recent availiable. If the schema version is already set (presumably from a previous installation) it is not changed.