Added (back) the '-d' command-line option to disable the deletion of processed
(e.g. imported) inbound netmail messages.
- This option has no effect if DeleteNetmail is set to "false" in sbbsecho.ini.
- This option *only* disables deletion of incoming netmail (no effect on
  outbound netmail deletion)
- In SBBSecho v2, the '-d' option disabled all netmail deletion, even though
  the help output only mentioned "netmail after import" (it was wrong).