slstatus: add cpu_iowait

slstatus: add cpu_iowait

This commit introduces the cpu_iowait item, this is the percentage
of cpu time spent waiting on disks. High numbers typically indicate
that your system is not responsive due to disk IO.
This commit also avoid sleeping inside the cpu_perc and cpu_iowait
functions: waiting in either one implies lost info for the other.

Signed-off-by: Kurt Van Dijck <[email protected]>