Fix the "default" tls-version-min setting

Fix the "default" tls-version-min setting

commit 968569f83b1561ea4dff5b8b1f0d7768e2a18e69
defined TLS 1.2 as the minimum version if not set
by user. But the patch introduced two errors:

(i) ssl_flags is overwritten without regard to other
    options set in the flags
(ii) Any tls-version-max set by the user is not taken into
     account.
Makes it impossible to set tls-version-max without also setting
tls-version-min along with loss of other bits set in ssl_flags.

Fix it.

The fix retains the original intent when possible, and tries to
use the maximum possible value when it cannot be set to TLS 1.2
without conflicting with user-specified tls-version-max, if any.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211015043227.10679-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22939.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>