Change the prerender code to run in the background so that Mike does
not have to wait 3 minutes for it to finish before he can watch his experiment swapin fail for some other reason. I adopted the same pid mechanism as in eventsys_control.in, which uses a slot in the experiments table. Running "prerender" puts the render into the background and stores the pid. Running "prerender -r" kills a running prerender and removes the existing info from the DB. Fixed the problem with swapmod not restoring the old vis; swapmod now kills any running prerender, and restarts one if the swapmod fails (the prerun of the new NS file starts up another prerender in the background). Add setpriority() call in prerender to nice it and children to 15.
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