Move to common backend_driver type in struct tuntap

Move to common backend_driver type in struct tuntap

With the introduction of utun on macOS and DCO on Linux, FreeBSD and
Windows, a lot of platforms have now more than one driver/backend for
the tun interface but each one uses a different mechanism. Unify these
approach with using a common enum that defines the driver_type.

Change-Id: I8c0e9f32b235cb262ca2be6aac8d520e49b30d74
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240922203749.9802-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29352.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>