ifpps: Allow to select number of top hitter CPUs by command line option

Instead of hard-coding the maximum number of top hitter CPUs, allow the
user to set it manually using a command line option. The default value
(if no command line parameter is specified) remains at 10.

Also update the manpage accordingly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>