Maildrop Spam Filter

This isn't so much a mdropspamfilter problem as a
client limitation, but is easy enough to fix here.

Some mail clients can't filter on the -existance- of a
header, they can only filter on text -in- that header.

As a workaround, it is nice to change all of
the "xfilter" message-tagging calls from "X-Antispam:
<message>" to "X-Antispam: spam <message>".

This way you can create a client-side filter that
says: "If header 'X-Antispam' contains 'spam' drop it
into the trash."