ifpps: show percentage of theoretical line-rate

Also show the percentage of the (theoretical) line rate, not just the
current rate.

Suggested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ifpps: make display of line rate percentage optional

In order to not to clutter the display too much, make the display of the
percentage of the theoretical line rate dependent on a command line
option.

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

Add a man page entry for the new -P option.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>