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

[Stefan Monnier]

> (file-acl "~/.emacs")

> "O:S-1-5-21-3955186872-1115707380-970145838-1000G:S-1-5-21-3955186872-1115707380-970145838-513D:(A;;FA;;;SY)(A;;FA;;;BA)(A;;FA;;;S-1-5-21-3955186872-1115707380-970145838-1000)"

This looks thoroughly unportable.

> (file-acl "/plink:albinus@ford:~/.emacs")

> "user::rwx
> group::rwx
> other::r-x
> "

We should probably change file-acl so that its return value is tagged
with the "acl-system" it uses.  It could be a prefix like "w32:".
Then we can change set-file-acl so that it can either try to perform
conversion or signal a proper error that copy-file can handle.


        Stefan