=head2 whitelist

Checks to make sure that the incoming message is from a poster in a file
of valid posters.  The most common use of this module is in conjunction
with human moderation; all messages from pre-approved posters are sent
straight through via the robomoderator and the rest are relayed to a human
moderator for hand checking.

This module takes one configuration directive:

=over 4

=item whitelist FILE

C<FILE> is a list of poster addresses, one per line.  Only the address
should be included in this file, not the name or other comments.  Files
ending in C<.db> are reserved for future implementations (eventually,
these will automatically be recognized as Berkeley db files for handling
large lists of posters).

=back

There is one possible failure message:

=over 4

=item Unknown poster %s

The message is from a poster address which isn't in the whitelist of known
addresses.

=back

In addition, during parsing of the configuration directives, one fatal
error, which would be passed to error(), is possible:

=over 4

=item Can't open whitelist file %s: %s

An error occurred while trying to open a file given as a C<FILE> argument
to a configuration directive.

=back

This module takes no arguments.