default blocking of GDAL VRT WCS WMS and MEM drivers

git-svn-id: http://svn.osgeo.org/postgis/trunk@12456 b70326c6-7e19-0410-871a-916f4a2858ee
by default, prevent PostGIS raster from using GDAL drivers that can easily be abused (VRT, MEM, WCS, WMS)

git-svn-id: http://svn.osgeo.org/postgis/trunk@12457 b70326c6-7e19-0410-871a-916f4a2858ee