Fix some s_server issues on Windows

In s_server we call BIO_sock_should_retry() to determine the state of the
socket and work out whether we should retry an operation on it or not.
However if you leave it too long to call this then other operations may
have occurred in the meantime which affect the result. Therefore we should
call it early and remember the result for when we need to use it. This fixes
a test problem on Windows.

Another issue with s_server on Windows is that some of output to stdout does
not get displayed immediately. Apparently more liberal use of BIO_flush is
required.

RT#4255

Reviewed-by: Richard Levitte <levitte@openssl.org>