radvd: Upsate 1.8.1 -> 2.5
nixos/radvd: Convert to a systemd unit

Additionally, remove the automatic initialization of the ipv6 forwarding
sysctl as this should be handled by the end user. This really should not
be an issue as most people running radvd are likely forwarding ipv6
packets.
cantata: update to 1.4.0
New Haskell package: network-fancy