This commit fixes the following bug:
When two or more default routes with metric 0 where in place, the update-status
hook fails to check that the default gateway is correctly set.

The update-status hook ensures that the configured default gateway is set.

Common functions between hooks have been moved to inc/common

The hook was looking for the default gateway in the machine's configuration
files, but this didn't work when using DHCP. Also, there is no real advantage
in modifying the machine's configuration files.

So we just change the routes at run time.