ifpps: Always display the least heavy hitter CPU

Along with the topmost heavy hitters, we always display the least heavy
hitter (unless there's only 1 CPU). This is not influenced by the -n
command line parameter, so also display the according additional CPU in
the header line.

Also update the manpage accordingly.

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