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.
tinc_pre: Add derivation

The tinc developer has made significant developments to the next version
of tinc including many protocol security fixes. This patch adds the
prerelease of the next major tinc release.