implement platform generic networking API

implement platform generic networking API

tun.c and route.c contain all the code used by openvpn
to manage the tun interface and the routing table on all
the supported platforms.

Across the years, this resulted in a longer functions
and series of ifdefs.

This patch introduces a new "networking API" which aims at
creating a simple abstraction between the tun/route logic
and the platform dependent code.

The is API expected to be implemented outside of tun.c/route.c
by using platform specific functionalities.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190514081159.12192-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18458.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>