get_default_gateway(): implement platform support for Linux/IPROUTE2

get_default_gateway(): implement platform support for Linux/IPROUTE2

Remove the old "read /proc/net/route and try to parse it" implementation
and always use the sitnl/netlink implementation of net_route_v4_best_gw().

This was kept "because we had it and it was working" but does not really
provide any benefit - netlink for route queries is there for v6 anyway,
and the main argument for keeping --enable-iproute2 is "some users want
to run non-standard 'ip' binaries to do things" - which is not affected
by this change.

Change-Id: I6f17140109106b37e6b0e690df1d87720ccf6f91
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20250131084707.24905-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30748.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>