gnutls: only translate winsock errors for old versions

Bugfix: https handshake fails using gnutls 3 on windows
http://sourceforge.net/tracker/index.php?func=detail&aid=3441084&group_id=976&atid=100976

New gnutls versions have an error handler that knows about Winsock
errors, which is why gnutls_transport_set_global_errno() was deprecated
and then removed.

This is a correction of commit f5bb3701 (blame me) which meant to
reimplement gnutls_transport_set_global_errno(), which is not necessary.