setup-etc.pl: Fail when symlink/rename fails

When atomicSymlink can't symlink or rename, it should return failure. This is then handled with `... or die` and `... or warn`

nixos/modules/system/etc/setup-etc.pl

···
     my ($source, $target) = @_;
     my $tmp = "$target.tmp";
     unlink $tmp;
-    symlink $source, $tmp or return 1;
-    rename $tmp, $target or return 1;
+    symlink $source, $tmp or return 0;
+    rename $tmp, $target or return 0;
     return 1;
 }