Print cumulative stats with -Q SECS,cumulative

Print cumulative stats with -Q SECS,cumulative

With -Q SECS, fping emits information for the just finished
interval.  This is good for long-running fping processes where
one would like to get an update on recent results, ignoring
missing responses from older intervals.  This seems appropriate
for use with other software like netdata or smokeping.

But for short(er)-running fping processes, e.g., during a change
window, it may be more interesting to get a status update since
the beginning of the change window, i.e., the start of fping.
Adding a comma followed by the keyword "cumulative" to -Q SECS
(i.e., -Q SECS,cumulative) changes the interim reports to output
the current per system overall statistics, unless -N is used, too.

The new syntax is documented in fping.pod and added to fping -h
output.

Four tests are added:

  1. Characters after SECS are ignored as before.
  2. Unknown keywords are ignored as before.
  2. Adding ",cumulative" changes -Q output.
  3. Adding ",cumulative" also affects the additional -o output.

This addresses issue #243.