save stderr to file in case it is useful

Author: mcr

tests/TESTonce

@@ -30,7 +30,7 @@ if ($^O eq 'MSWin32') {
 else {
  # we used to do this as a nice pipeline, but the problem is that $r fails to
  # to be set properly if the tcpdump core dumps.
- $r = system "$TCPDUMP 2>/dev/null -# -n -r $input $options >NEW/$outputbase";
+ $r = system "$TCPDUMP 2>NEW/$outputbase.stderr -# -n -r $input $options >NEW/$outputbase";
  if($r != 0) {
  my $coredump = false;
  my $status = 0;