reference doc about ACLs

[Bruno Haible]

The copy_file function, used by GNU gettext, attempts to copy the ACLs from
the source file to the destination file. The 'acl' module is not well
maintained currently: it yields an error message on MacOS X 10.5 and fails
to copy the ACLs on HP-UX 11 and possibly also on Solaris 10.

Before I can do any modification on it, I needed to get an overview of the 
topic.
Essentially, every system does it differently:
  Linux
  Solaris
  FreeBSD
  MacOS X
  HP-UX
  IRIX
  Tru64
  AIX
  AIX >= 5.3
  Cygwin
  mingw
Five among these systems have similar API, based on an old POSIX draft that
was withdrawn (!).

2008-05-21  Bruno Haible  <address@hidden>

        Add reference doc about ACLs.
        * doc/acl-resources.txt: New file.
        * doc/acl-cygwin.txt: New file.