Fix a slight race condition that has already bitten us several times in
real life. There's a small window in time (typically a few seconds, but maybe 10's of seconds on occasion) between the time nodes get nalloc'd and the time they get rebooted during a swapin. If idlemail runs (or someone views a web page) during this time, the expt can appear idle because the nodes haven't had any activity. It can cause an incorrect email or a false positive on the web page. The fix is to have nalloc register some external node activity, like node_reboot does, when it allocates a node.