gdal_edit.py: add a -ro option for drivers refusing to use the dataset in update-mode. In which case, updated information will go into PAM .aux.xml files

git-svn-id: https://svn.osgeo.org/gdal/trunk@26564 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx