Revision 4942

backport (glarose): Add support for external authentication systems.

Add support for external authentication systems.    This is appropriate
if something like an apache directory or location based authentication
system (such as cosign, basic or digest auth) is used.  In these cases,
WeBWorK may not handle the logout procedure, and we need information
from the Authentication module to determine the correct action.  We
allow for the case of redirects here, but this could easily be
extended if needed.

Implementation: pre_header_initialize() checks for $authen->{redirect}
to be defined and true after calling $authen->forget_verification().
If to, we set $self->reply_with_redirect to the value of returned by
$authen->{redirect}.

This update also moves key deletion from body() to
pre_header_initialize(), so that those actions take place before any
redirect.