nixos/network-interfaces: Add sit interfaces

Previously, we had no method for creating 6-to-4 tunneled interfaces.
This patch adds the option networking.sits, which allows the user to
create named 6-to-4 sit devices.
nixos/dhcpcd: Don't configure sit devices
Merge pull request #3087 from wkennington/master.sit

nixos/network-interfaces: Add sit interfaces