Route: add support for user defined routing table

Route: add support for user defined routing table

Add the ability for users to specify a custom
routing table where routes should be installed in.
As of now routes are always installed in the main
routing table of the operating system, however,
with the new --route-table option it is possibile
to specify the ID of the default routing table
to be used by --route(-ipv6).

Please note: this feature is currently supported
only by Linux/SITNL.
Support for other platforms should be added in related backends.

Trac #1399
Change-Id: I3e4ebef484d2a04a383a65ede5617ee98bf218a7
Signed-off-by: Gianmarco De Gregori <gianmarco@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250622110311.1140-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31946.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>