Add GDAL_DCAP_OPEN so that we can distinguish write-only drivers

git-svn-id: https://svn.osgeo.org/gdal/trunk@27487 f0d54148-0727-0410-94bb-9a71ac55c965
Convert PGDUMP and ElasticSearch as GDAL drivers

git-svn-id: https://svn.osgeo.org/gdal/trunk@27488 f0d54148-0727-0410-94bb-9a71ac55c965