As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.
Motivation: https://github.com/NixOS/nixpkgs/issues/112647
This option allows users to specify a local UNIX control socket to
"remote control" the daemon. System users, that should be permitted to
access the daemon, must be in the `unbound` group in order to access the
socket. When a socket path is configured we are also creating the
required group.
Currently this only supports the UNIX socket mode while unbound actually
supports more advanced types. Users are still able to configure more
complex scenarios via the `extraConfig` attribute.
When this option is set to `null` (the default) it doesn't affect the
system configuration at all. The unbound defaults for control sockets
apply and no additional groups are created.
