ifpps: Calculate and show average values across all CPUs

Calculate the average for CPU usage, IRQs/t and absolute IRQs across all
CPUs (also the ones which are not displayed) and show them below the
respective per-CPU values.

Mention this fact the man page accordingly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ifpps: Reduce default number of top hitter CPUs to 5

Since we now also display the average (and in the future possibly also
the standard deviation), reduce the default number of CPUs displayed to
5, to keep terminal space usage low.

Suggested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>