df: new option: --total to print grand totals

df: new option: --total to print grand totals

* src/df.c (add_uint_with_neg_flag): New function to add two integral
values with separate negation flag.
(show_dev): New parameter force_fsu to display numbers directly. Collect
summary statistics on each printed device.
(usage): Mention new option --total in --help.
(main): Initialize summary on program start. Handle new option --total.
* tests/df/total: Dummy test case for new --total option.
* tests/df/total-awk: Better test case for new --total option (requires
awk).
* doc/coreutils.texi: Mention new parameter --total.
* NEWS: Mention the change.
* TODO: Removed completed task.