Don't overwrite socket flags when using DCO on Windows

Don't overwrite socket flags when using DCO on Windows

Socket flags can be pushed, in which case they overwrite
existing value. We use socket flags to distingust between
DCO handle and socket on Windows. If server pushes --socket-flags,
we treat DCO handle as socket and everything explodes.

Fix by making link_socket_update_flags() update flags
(like name suggests) instead of overwriting them. Also
do not set TCP_NODELAY on DCO handle on Windows because
it doesn't make sense.

Change-Id: Ia34d73ca49041cb0ce22b84751cdbff57de96048
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230324121818.2358-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26513.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 82e7d5cfd81f03f045ace2bf1d3590b79441ea17)