uniq: add the --group option
* src/uniq.c (usage): Summarize the new option,
and adjust the --all-repeated option to be more consistent.
(check_file): Merge the --group functionality into
the core loop for the default uniq operation since
it's very similar and can output lines immediately upon reading.
(main): Handle the new --group option and make it
mutually exclusive with other selection options.
* tests/misc/uniq.pl: Add tests.
* NEWS: Mention the new feature.
* doc/coreutils.texi (uniq invocation): Describe --group.