This commit adds an IPv4Gateway to Network.ini. If that option is set to
value other than 0.0.0.0, the NetworkServer adds a default route (e.g.
with address 0.0.0.0/0) with the specified destination.
This service is responsible for loading network configuration from a
/etc/Network.ini config file. It sets up static IP address + mask or
starts DHCPClient depending on configuration.
