copy_network_params() improvements
- Define the MTU for every interface, including the main one
- Allow to define a default MTU value to use for every interface without an explicit value (DEFAULT entry)
- Allow to define a value for the main interface, independent of its names (BOOT entry)
- Allow to define a value based on the interface type (e.g. 'eth', 'em')
- Allow to let MTU size undefined using value undef to define an exception to previous rules)