Simple PAM module to check the username's existance (or lack of) in a file.

To use simply call from a PAM file passing the file to check (file=) and the
mode (mode=).  If mode is block then a user is authenticated if they don't
exist in the file.  Any other mode means the user is authenticated if they do
appear in the file, e.g.

auth       required    pam_file.so         file=/etc/email.allow mode=allow