ifpps: Don't re-initialize stats value index counter in CSV mode

In case we add new stats values, j might be higher already when entering
the loop. As j will have the proper number anyway, there is no point in
re-initializing.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ifpps: Don't diff tx_bytes twice

It's sufficient to calculate the difference of tx_bytes only once.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ifpps: Don't calculate diff for stats values not used for relative values

These stats values are not reported anywhere in relative terms, so it
makes no sense to calculate the difference for them.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ifpps: Report more detailed memory stats and number of total processes

Instead of just giving the percentage of memory usage, report the total,
used, active and inactive memory values. Also report total, free and
cached for swap.

Instead of reporting the memory in the 'sys' line add two new lines for
these reports and use the free space to report the total number of
processes.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
man: ifpps: Reword a few sentences or make them shorter

Fix some minor nits in the description part and remove a superfluous
comma in the description of option -d/--dev.

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