netsniff-ng: omit '-' char in dump prefix

netsniff-ng: omit '-' char in dump prefix

Based on user request:

  Thanks for all the fixes lately! I've got the latest git version
  packaged and it looks like we're making great progress!

  I'm running as follows:
  sudo netsniff-ng -i eth0 -o test/ --interval 1MiB -Psnort.log -s

  In my test directory, I have filenames like:
  snort.log-1352859267.pcap

  However, Sguil is expecting the filenames to be like this:
  snort.log.1352854983

  Would you mind fixing this format or making the format more
  configurable? Or should I just do a quick-and-dirty hack?

Reported-by: Doug Burks <doug.burks@gmail.com>
Signed-off-by: Daniel Borkmann <borkmann@iogearbox.net>