CPLsetlocale(): return a string that is thread-locale storage to avoid potential race in CPLLocaleC::CPLLocaleC() (#5747)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27997 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx