Fix bug #13298 with failed backups by falling back on set-file-modes.
src/fileio.c (Fset_file_selinux_context, Fset_file_acl): Return t if
file's SELinux context or ACLs successfully set, nil otherwise.
lisp/files.el (backup-buffer-copy, basic-save-buffer-2): If
set-file-extended-attributes fails, fall back on set-file-modes
instead of signaling an error.
doc/lispref/files.texi (Changing Files): Document the return values of
set-file-selinux-context and set-file-acl.