Run prelink and other stuff not in cron but when screensaver is active.

[Jaap A. Haitsma]

I sometime get a bit frustrated that a cron job starts while I'm 
working. Wouldn't it be nicer that this stuff would run when I'm not 
doing any work. I.e. when the screensaver is active. Maybe as a minor 
enhancement when the screensaver is active and there is not a process 
asking for cpu power /disk access. Sometimes I'm running compiling jobs 
which take quite a while.

Jaap