exit codes in docs

[Jeff Flowers]

> Actually, the whole exit code interface was misdesigned from
> the moment it sprang into being. A program takes standard input
> and prints its finding on standard output. Using exit codes to
> differentiate between "no error" results is wrong.
>
> > I strongly agree here. We are still at some more or less
> > early stage of development. Fixing everything too early
> > creates problems. Changes create problems. So as someone
> > else said: If we do need to make changes, let's do it now.
>
> My suggestion is remove the "exit code" interface and just use an
> interface along the lines that bogofilter -tv or bogofilter -p
> provide.

Unlike Matthias, I think that the exit codes are a good way of doing things
and I want their use retained in current and future versions of Bogofilter.
I do agree, however, that it would be best to leave the current exit codes
as is and add "unsure" to exit code 3.

Jeff