Fix intermittent Cygwin failures in s_client

This was identified for Windows almost two years ago for VC and
msys/mingw.  It seems that Cygwin suffers from the same issue, and
since Cygwin doesn't define OPENSSL_SYS_WINDOWS, we need to make a
special case to have a 50ms pause before closing the TLS connection.

Ref: git commit cb2e10f2



Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5074)