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

checkfunc is mainly replaced by doku_hasTextBeenModified, which is put
in  global scope to allow overriding by plugins.

In php7 constructors with the same name as the class are deprecated. __construct() should be used.

Depending on the server-configuration this could result in a warning which would break the API.