Restart nscd (if running) when interface comes up

DNS entries can become stale when switching networks. We need to invalidate nscd's hosts cache when a new network (interface) comes up. The simplest way to do this on NixOS is to simply restart the nscd service. This is what other distros running NetworkManager do; see e.g. http://lists.freedesktop.org/archives/systemd-devel/2012-April/004877.html