Avoid errors in upgrade path

Running the upgrade path from Drupal 5 to 6 currently throws about a dozen errors on my sandbox sites. It looks like the culprit is hook_menu() firing during the update whenever the cache is cleared, which then in turn fires content_fields() which eventually queries on the "active" column in the field database table (which doesn't yet exist).

Strangely, hook_menu() for CCK already has an IF statement that is supposed to keep (at least some of) the errors from occurring. This patch sidesteps using any functionality in CCK if we're currently in maintenance mode (on either update.php or install.php), rather than doing any checks by CCK's functions.