Distinguish route addition errors from route already exists

Distinguish route addition errors from route already exists

When possible, functions that add a route now return 1 on success,
or 2 if route already exists or 0 on other errors instead of true/false.

Note:
net_route_v4/v6_add using netlink filters out EEXIST before returning
this looks like a bug as add_route() and add_route_ipv6() should set
RT_ADDED only if route was really added.

v2: "succeeded/skipped" --> "succeeded" in log.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230106150412.1667492-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25903.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>