Refactoring BIO: add a simple networking test of s_client and s_server

This makes use of TLSProxy, which was expanded to use IO::Socket::IP
(which is a core perl module) or IO::Socket::INET6 (which is said to
be more popular) instead IO::Socket::INET if one of them is installed.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>