[development] Temporary database credentials

[Moshe Weitzman]

> It would be nice to have a place to enter some temporary credentials, 
> stored in $_SESSION and disposed of when the user logs out.


Interesting. Perhaps we should let users specify a different $db_url in 
settings.php that is used while updating. If not specified, we use the 
usual $db_url. How about this pseudo-code in update.php?

global $user, $db_url, $db_url_updating
if ($user->uid == 1) {
   $db_url = $db_url_updating;
}