ogr2ogr: add -addfields option to add new fields found in a source layer into an existing layer ; add -unsetFieldWidth option to unset field with and precision; add -dim layer_dim option to force the coordinate dimension of geometries to match the one of the layer geometry type

git-svn-id: https://svn.osgeo.org/gdal/trunk@26141 f0d54148-0727-0410-94bb-9a71ac55c965