PG: add a OGR_TRUNCATE configuration option that can be used as an alternative to the -overwrite flag of ogr2ogr, but that avoids DROP TABLE ... CASCADE which destroys views based on the tables (#5091)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26055 f0d54148-0727-0410-94bb-9a71ac55c965