Extend action plugin to skip login actions and improve user registration

This change does three things:
1. The code is now clearly separated between the auth and the action plugin:
   if authhttp is used standalone as DokuWiki's auth plugin, only the auth
   plugin will have visible effect. When used together with authsplit, with
   authhttp being the primary auth plugin, the action plugin will show effect
   and make authhttp integrate more nicely with authsplit.
   In order to make the code cleaner, the action plugin now also has a
   __construct() method that does things necessary when authsplit is active,
   such as disabling certain DokuWiki features.
2. The action plugin now registers a second event handler that gets called
   after ACTION_ACT_PREPROCESS and will make sure that any 'login' action gets
   redirected to the default 'show' action instead. Since it does not make
   sense to show a login form when HTTP authentication is used. (If authhttp
   is used standalone, a user will ALWAYS be logged in, so this won't have any
   effect.)
3. The event handler for HTML_REGISTERFORM_OUTPUT now makes sure that DokuWiki
   gets a random password for creating the user account. That password won't
   get stored anywhere thanks to authsplit but the register() code insists on
   a non-null password, so there.
Update README and plugin info