comm: add --total option

comm: add --total option

* src/comm.c (total_option): Add bool variable for the new option.
(TOTAL_OPTION): Add enum value.
(long_options): Add array element for the new option.
(usage): Document the new option here.
(compare_files): Count the lines in total[3], and output the summary at
the end.
(main): Accept the new option.
* doc/coreutils.texi (comm invocation): Document it.
* tests/misc/comm.pl: Test it.  While at it, improve the test data
to have 1 unique line in the first file, 2 unique lines in the second
file, and 3 common lines.
* NEWS (New Features): Mention the new option.

Fixes http://bugs.gnu.org/24929