Add a new syscall latnice for "latency nice" with 4 levels, 1 for default, 0 for...

Add a new syscall latnice for "latency nice" with 4 levels, 1 for default, 0 for latency insensitive, 2 for relatively latency sensitive gui apps, and 3 for exquisitely latency sensitive uses such as audio. Also allow the user to choose a level for desktop interactivity with a /proc/sys/kernel/interactive tunable. Full explanation at ck-hack.blogspot.com/2010/12/automated-per-session-task-groups.html