Don't assume QLocale::codecForLocale always returns non-null

It may return null during program exit, due to QCoreGlobalData global
static already having been destroyed. If that's the case, QTextStream
needs to fall back to Latin 1, like QString::toLocal8Bit and
fromLocal8Bit already do.

Task-number: QTBUG-38316
Change-Id: I5949c8dec15b60f4a13b5d9307ed6abfc799fe20
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Doc: change the name of the QTemporaryDir parameter name

Make it very clear that this is a path, so it's relative to the working
dir, not relative to tempPath().

Task-number: QTBUG-38266
Change-Id: Ib7ca8df76b5a03c1631fe00d6b329d86538d4b5a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>