Impact: cleanup, change .config option name

We had this ugly config name for a long time for hysteric raisons.
Rename it to a saner name.

We still cannot get rid of it completely, until /proc/<pid>/stack
usage replaces WCHAN usage for good.

We'll be able to do that in the v2.6.29/v2.6.30 timeframe.
Signed-off-by: Ingo Molnar <mingo@elte.hu>