Allow auth->getUserData to be retrieved with or without group memberships

Also implements caching of user data for mysql & pgsql plugins.

Resolves [FS#2086](https://bugs.dokuwiki.org/index.php?do=details&task_id=2086)
Addresses https://github.com/splitbrain/dokuwiki/issues/587
There is this function missing as well.
I guess it should be _getUserInfo(), but I haven't really looked into it.