- change the use of $ENV{HOSTNAME} by `hostname`

- Put the flag "S" if status is "O" (used by uw-imap/pop)
- write to /dev/null if $header is empty (avoid null messages)
- don't increment $num if message is going to be written to /dev/null...
  (null message or "DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA"
  message)