Refactored auth system: All auth methods are now introduced as plugins.

inc/auth.php

inc/auth/basic.class.php

inc/auth/pgsql.class.php

lib/plugins/auth.php

lib/plugins/authad/auth.php

lib/plugins/authad/plugin.info.txt

lib/plugins/authldap/auth.php

lib/plugins/authldap/plugin.info.txt

lib/plugins/authmysql/auth.php

lib/plugins/authmysql/plugin.info.txt

lib/plugins/authpgsql/auth.php

lib/plugins/authpgsql/plugin.info.txt

lib/plugins/authplain/auth.php

lib/plugins/authplain/plugin.info.txt

lib/plugins/config/settings/extra.class.php