Route: remove incorrect routes on exit

Route: remove incorrect routes on exit

Implemented a safeguard to verify the returned value
from add_route3() when the default gateway is not a local
remote host.

Prior to this implementation, RT_DID_LOCAL flag was
erroneously set even in case of add_route3() failure.
This problem typically occurs when there's no default
route and the --redirect-gateway def1 option is specified,
and in case of reconnection makes it impossible for the client
to reobtain the route to the server.
This fix ensures OpenVPN accurately deletes the appropriate
route on exit by properly handling add_route3() return value.

Trac: #1457
Change-Id: I8a67b82eb4afdc8d82c5a879c18457b41e77cbe7
Signed-off-by: Gianmarco De Gregori <gianmarco@mandelbit.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240221111814.942965-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28290.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 14d2db6cd41fb6414992869caf109972d7a8275e)