ssl_common: Make sure ssl flags are treated as unsigned

ssl_common: Make sure ssl flags are treated as unsigned

tls_options.ssl_flags is already unsigned, make sure the
flags are as well to avoid spurious conversion warnings.

Also fix various warning regarding the use of the flags
for TLS version handling.

Change-Id: I03e5ece7580ca4ebd41a7928ead544df46e8bad1
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: MaxF <max@max-fillinger.net>
Message-Id: <20250911201658.25736-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59232184/
URL: https://gerrit.openvpn.net/c/openvpn/+/1104
Signed-off-by: Gert Doering <gert@greenie.muc.de>