After the systemd 237 upgrade, radvd wouldn't start anymore because the
PID file cannot be written. It seems that directories in /run has to be
explicitely defined as RuntimeDirectory now. The PID file isn't needed
due to systemd, though, so it was removed along with forking and loggia
via syslog.
This fixes the ipv6 NixOS test.
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.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
