add option to use user defined routing table

By using this option the user can decide which routing table should be used by OpenVPN.

Benefits:
1. No need for dedicated up/down scripts to archive  that behavior 
2. cleaner routing setup / dedicated tables for different instances
3. Ease of use - mark all packets that should use the tunnel using an iptables rule and
apply an ip rule that all marked packets should use the routing table
containing the route entries for the/that tunnel
4. By using (3) it is possible to switch the packet flow between vpn tunnel and other routing 
targets (eg. direct connection, second tunnel, etc.) without changing the routing tables
5. By using (3) killing the tunnel client means also interrupting the connection for the tunneled 
clients (having the default routes mixed with the route entries from OpenVPN results in a
fall back to the former routes if the OpenVPN client gets (accidentally) killed and that may lead
to a security problem).

Draw backs: 
This option works only on LINUX with iproute2 support enabled (enable_iproute2).