Added support for "on-link" routes on Linux client

Added support for "on-link" routes on Linux client

These are routes where the gateway is specified as an interface rather
than an address. This allows redirect-gateway to work on Linux clients
whose connection to the internet is via a point-to-point link such as
PPP.

Note that at the moment, this capability is incompatible with
the "redirect-gateway block-local" directive -- this is because
the block-local directive blocks all traffic from the local LAN
except for the local and gateway addresses.  Since a PPP link
is essentially a subnet of two addresses, local and remote (i.e.
gateway), the set of addresses that would be blocked by block-local
is empty.  Therefore, the "redirect-gateway block-local" directive
will be ignored on PPP links.

To view the OpenVPN client's current determination of the default
gateway, use this command:

  ./openvpn --show-gateway

git-svn-id: http://svn.openvpn.net/projects/openvpn/branches/BETA21/openvpn@7794 e7ae566f-a301-0410-adde-c780ea21d3b5
Signed-off-by: James Yonan <james@openvpn.net>
Signed-off-by: David Sommerseth <davids@redhat.com>