This is a DKIM signing and verification milter.  In theory it works with both
Postfix and Sendmail, but the author has zero experience with Sendmail, so
reports of success/failure with Sendmail and patches are welcom.

The configuration file is designed to be compatible with OpenDKIM, but only
a subset of OpenDKIM options are supported.  If an unsupported option is
specified, an error will be raised.