[Mimedefang] Starting work on adding filter_helo to mimedefang

[Philip Prindeville]

I remember seeing something that there's a bug in sendmail that might
stop a "filter_helo" from working.  Anyone have the specifics of this?
Like what the behavior is, and in what version of sendmail it was
fixed?

I'm running 8.13.1 on FC3, and Mimedefang 2.54.  I'll post the patch
for testing when I'm satisfied it's stable.

-Philip