bug#13226: 24.3.50; set-file-acl on MS Windows shall check ACL string format

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Michael Albinus <michael.albinus@gmx.de>,  romain@orebokech.com,  
> 13226@debbugs.gnu.org
> Date: Wed, 19 Dec 2012 13:36:43 -0500
> 
> I don't think we should document that file-acl returns a string.
> The representation should be documented as undocumented and prone to
> change (ideally opaque).

Currently, it could be either a string or nil.  The latter means "no
ACL".  Do you want to undocument both of these possibilities?  It
sounds too radical to me.