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>
ifpps: Also display min hitter for IRQs

Also display the least heavy hitter for the relative and absolute IRQ
list. Also check that the number passed with -n is actually 1 or larger.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Merge remote-tracking branch 'upstream/master'