uniq: fix bug with -w in multibyte locales

uniq: fix bug with -w in multibyte locales

-w counted bytes not characters, which is wrong in multibyte locales.
This bug exists even in Fedora, which is why the recently-added
test cases from Fedora didn’t catch it.
* src/uniq.c (find_field): New arg PLEN.  All callers changed.
Compute length of field correctly in multi-byte locales.
(different): Don’t worry about check_chars; find_field now does that.
* tests/uniq/uniq.pl: Test for this bug.