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 f19b58fb6a)
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).
