fixing dhcp configuration

- if two or more interfaces are configured with 'inet dhcp' and ifdown/ifup is called for other interfaces than eth0, dhclient requests/releases leases for all interfaces. as a workaround set interfaces to 'manual' and call dhclient with different configuration files
- adding default routes for each interface to get ping to e.g. eth1 working (see http://kindlund.wordpress.com/2007/11/19/configuring-multiple-default-routes-in-linux/)