Patch by Andreas Barth: added --secret-keyring /dev/null

Patch by Andreas Barth: added --secret-keyring /dev/null

This is required for ud-mailgate to work with sarge as well.  The
semantics of the option --no-default-keyring of GnuPG has been changed
between woody and sarge:

--no-default-keyring
  Do not add the default keyrings to the list of keyrings.  Note that
  GnuPG will not operate without any keyrings, so if you use this
  option and do not provide alternate keyrings via --keyring or
  --secret-keyring, then GnuPG will still use the default public or
  secret keyrings.  (and won't work if they don't exist)