tr: do not reject an unmatched [:lower:] or [:upper:] in SET1.

tr: do not reject an unmatched [:lower:] or [:upper:] in SET1.

* NEWS: Mention this.
* src/tr.c (get_next) [RE_CHAR_CLASS]: Don't skip the loop when
processing [:lower:] and [:upper:].
(main): Require [:lower:] or [:upper:] in SET1 only when
when one of those is specified in SET2.
* tests/tr/Test.pm: Add tests for this fix.
Reported by Per Starbäck.

Signed-off-by: Jim Meyering <meyering@redhat.com>