tun.c: use new networking API to handle tun interface on Linux

tun.c: use new networking API to handle tun interface on Linux

By switching to the networking API (for Linux) openvpn will
now use any of the available implementations to handle the tun
interface.

At the moment only iproute2 and sitnl (NetLink) is implemented.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20181219050118.6568-4-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18028.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>