The networkd implementation sets systemd.services.dhcpcd.enable to
false in nixos/modules/tasks/network-interfaces-systemd.nix. So we need
to respect that in the dhcpcd module.
If we don't, the resumeCommand is set nevertheless, which causes the
post-resume.service to fail after resuming:
Failed to reload dhcpcd.service: Unit dhcpcd.service is masked.
post-resume.service: main process exited, code=exited, status=1/FAILURE
Failed to start Post-Resume Actions.
Dependency failed for Post-Resume Actions.
Unit post-resume.service entered failed state.
post-resume.service failed.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>