TLSProxy/Proxy.pm: bind s_server to loopback interface.

Bind even test/ssltest_old.c to loopback interface. This allows to avoid
unnecessary alerts from Windows and Mac OS X firewalls.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5933)