networking: add net_iface_type API

networking: add net_iface_type API

This new API can be used to retrieve the type of a specific interface.
It's mostly platform dependant, but right now expected values are
"ovpn-dco", "tun" or "tap".

Other values are possible too, but they are not of interest to us.

This commit also extends the networking unit-test by using the newly
introduced API in conjunction with iface_new and iface_del.

The t_net.sh script has been slightly adapted to allow running these
tests in standalone (as they don't require any iproute2 counterpart).

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220713124332.16147-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24688.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>