KML: in writing mode, avoid defining an extending schema for the name and description fields (related to #5208)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26346 f0d54148-0727-0410-94bb-9a71ac55c965
ogr2ogr: check that detection of duplicated field names is case insensitive (#5208)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26348 f0d54148-0727-0410-94bb-9a71ac55c965