s_client: Add basic proxy authentication support

1) Add two new flags (-proxy_user & -proxy_pass) to s_client to add support for basic (base64) proxy authentication.
2) Add a "Proxy-Connection: Keep-Alive" HTTP header which is a workaround for some broken proxies which otherwise close the connection when entering tunnel mode (eg Squid 2.6).

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7975)