pidstat stack utilization statistics were not always properly refreshed

Stack utilization statistics displayed by "pidstat -s" were sometimes
not displayed for some processes although values had changed.
This was because stack stats were displayed together with memory stats,
but pidstat was looking only for a variation in memory counters values
to decide whether to display the corresponding process or not.
So now process memory and stack statistics independently.

Signed-off-by: Sebastien GODARD <sysstat@orange.fr.fake>