Avoid tr case-conversion failure in some locales.

Avoid tr case-conversion failure in some locales.

* src/tr.c (skip_construct): New function.
(main): When processing a pair of case-converting classes, don't
iterate through the elements of each [:upper:] or [:lower:] class.
Reported by Gerald Pfeifer in
<http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/12218>.
* tests/tr/Test.pm [tolower-F]: New test for the above fix.
[upcase-xtra, dncase-xtra]: New tests, for a related code path.
* NEWS: Mention the tr bug fix.