networking: implement net_iface_new and net_iface_del APIs

networking: implement net_iface_new and net_iface_del APIs

These two new methods can be used to create and delete a tun or an
ovpn-dco interface via networking API.

Implementations for SITNL and iproute2 are provided

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220407094146.7684-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24088.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>