Correct output order. Network gateways must output first

This is to correct an issue that came up in Rackspace's internal OpenStack system with CoreOS not having properly configured routing.

The order of routes in /etc/conf.d/net isn't guaranteed. It may be that the default route ("default via x.x.x.x") may come after unicast routes ("x.x.x.x netmask y.y.y.y gw z.z.z.z"). The gateway of the default must be included with the `[Network]` section, i.e., output before any `[Route]` sections. This pull request adds this correction.

I also changed the mix of tabs and spaces to just spaces. Let me know if this is unwanted and I'll update accordingly.
 
This issue was discussed with Michael Marineau.