Fix bug #13298 with failed backups by falling back on set-file-modes.

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.