Fix freopen handling of ,ccs= (bug 23675)

As reported in bug 23675 and shown up in the recently added tests of
different cases of freopen (relevant part of the test currently
conditioned under #if 0 to avoid a failure resulting from this bug),
freopen wrongly forces the stream to unoriented even when a mode with
,ccs= is specified, though such a mode is supposed to result in a
wide-oriented stream.  Move the clearing of _mode to before the actual
reopening occurs, so that the main fopen implementation can leave a
wide-oriented stream in the ,ccs= case.

Tested for x86_64.