This is a refactor of how resolvconf is managed on NixOS. We split it
into a separate service which is enabled internally depending on whether
we want /etc/resolv.conf to be managed by it. Various services now take
advantage of those configuration options.
We also now use systemd instead of activation scripts to update
resolv.conf.
NetworkManager now uses the right option for rc-manager DNS
automatically, so the configuration option shouldn't be exposed.
Rather than special-casing the dns options in networkmanager.nix, use
the module system to let unbound and systemd-resolved contribute to
the newtorkmanager config.
This shuts up this error from dbus:
May 11 13:52:16 machine dbus-daemon[259]: Unknown username "systemd-network" in message bus configuration file
May 11 13:52:16 machine dbus-daemon[259]: Unknown username "systemd-resolve" in message bus configuration file
which happens because the D-Bus config for networkd/resolved is
enabled unconditionally, and we don't have an easy way to turn it off.
(cherry picked from commit f19b58fb6a5cc55af6d8d8ca7979f8e64255d2d0)
Otherwise, the enabled -> disabled transition won't be handled
correctly (switch-to-configuration currently assumes that if a unit is
running and exists, it should be restarted).
