pidstat: Display stats since boot time for a list of given processes

pidstat displays statistics since system startup when the interval and
count parameters are not set on the command line (eg. entering "pidstat
-d" will display I/O statistics for all processes that have had I/O
activity since boot time). But pidstat couldn't display those stats when
some PID numbers were entered on the command line (eg. "pidstat -d -p
1234" to display I/O stats since system startup for process 1234).
This patch makes it possible now.

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