Return SELinux context of file named FILENAME.
The return value is a list (USER ROLE TYPE RANGE), where the list
elements are strings naming the user, role, type, and range of the
file's SELinux security context.
Return (nil nil nil nil) if the file is nonexistent or inaccessible,
or if SELinux is disabled, or if Emacs lacks SELinux support.
(fn FILENAME)