Remove support for rlogind in login(1), that is, remove the '-r' flag

Remove support for rlogind in login(1), that is, remove the '-r' flag

The "quick hack" finally disappeared.  Probably nobody noticed.  ;)
(See the changes in <configure.ac> for the context of this pun.)

Probably everybody uses SSH these days for remote login.  Let's remove
this insecure method.

Closes: <https://github.com/shadow-maint/shadow/issues/992>
Reviewed-by: dkwo <nicolopiazzalunga@gmail.com>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Michael Vetter <jubalh@iodoru.org>
Cc: Sam James <sam@gentoo.org>
Cc: Benedikt Brinkmann <datacobra@thinkbot.de>
Signed-off-by: Alejandro Colomar <alx@kernel.org>