We decoded the wrong option here before.

It seems some update of the adLDAP library made a crucial function
protected, thus breaking the authad plugin.

This just makes the function public again, which should fix the
immeadiate problem. A proper fix would need to do #1553 as well

See https://github.com/splitbrain/dokuwiki/issues/1535#issuecomment-227039700

Otherwise you have to reapply the filtering every time you updated one
plugin.

See #1551

Changed the values recognized by the general config parser.
If value in config is a boolean it is recognized as boolean, not if it
is a string.

Fixes #718