uniq: avoid strcoll() to improve performance and consistency

uniq: avoid strcoll() to improve performance and consistency

strcoll() is only significant to uniq(1) if it returns 0,
and it generally only does so with buggy locales or mismatched
locales and data.  Some systems may have strcoll()
return 0 for equivalent normalized unicode forms,
but for consistency across platforms strcoll() is avoided.
The various cases are defined in the new test.
This is consistent with newer POSIX standards as discussed at:
https://www.austingroupbugs.net/view.php?id=963

* src/uniq.c: s/xstrcoll/memcmp/.
* tests/local.mk: Reference the new test.
* tests/misc/uniq-collate.sh: Add a new test.
* NEWS: Mention the change in behavior.
Fixes https://bugs.gnu.org/38627