mailscanner: ability to run single message from command line

[Matthias Klose]

forwarded from http://bugs.debian.org/177027

It would be useful to include an option to the mailscanner program that
would read a single message from STDIN, process it, and return the
results to STDOUT.  Occasionally, mailscanner is marking a message
as spam (says it found it in razor2) but when I run the message
through "razor-check" or "spamassassin -t", then it lists no such
match.

Since I can't be sure that the body I tried to check was the same one
that mailscanner first looked at, I can't really proceed in tracking
down the source of the problem.