pidstat: Add an option to filter processes by name

Add "-G <process_name>" option to display only processes whose
name matches <process_name> regular expression.
While -C option filters both process and thread names, -G option
filters only process names, and displays all threads of matching
processes.

Signed-off-by: Cedric Marie <cedric.marie@openmailbox.org>
Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>