This is a major change to the system, needs more testing!

Committing Changes by Moshe Weitzman:
- admin_user_account(), user_edit(), and user_view() no longer have any
  hard code for authentication modules. instead authentication modules
  implement the _user hook.
- fixed a couple 'help' typos.
- linked the 'REGISTER' text in the login block to the register page.
  this page now advertises DA better if site employs DA.
- admins may now edit everything about a user account (was a feature
  request).
- user #1 may now login immediately, in addition to receiving his
  password via email.

Other changes:
- modules and themes are now enabled/disabled in the administrative /
  settings / modules | themes pages. Requires SQL update and things must
  be enabled before your site returns to normal. TODO: enable all
  functionality. (For now just do UPDATE system SET status = 1;)
- removed $themes from conf.php.
- added a $theme->system() function where theme can specify settings.
  All themes in the Drupal CVS have been updated to use this.
- added _system hook to modules. TODO: update modules to use this.
- changed strange use of sprintf to the usual strtr. The disadvantage of
  sprintf is that it requires translations to keep the string order,
  which may not be possible in all languages.
- an invalid/nonexisting theme in a user profile will now fallback to the
  BaseTheme instead of crashing.