Do traffic accounting in pps

Add a "-u unit" CLI option, as well as a "bandwidth-unit" configuration file
option. With "-u packets", traffic is accounted using packets per second; the
other options are "-u bits" and "-u bytes".

"-B" is still recognized as synonym to "-u bytes".

The default is "-u bits", keeping the current behaviour of iftop (everything
is in bits/s, except the cumulative totals).