initial steps at GUCs and additional drivers to skip by default

git-svn-id: http://svn.osgeo.org/postgis/trunk@12473 b70326c6-7e19-0410-871a-916f4a2858ee
Addition of GUC postgis.gdal_enabled_drivers with default set to all GDAL drivers disabled

git-svn-id: http://svn.osgeo.org/postgis/trunk@12474 b70326c6-7e19-0410-871a-916f4a2858ee
document the GUC postgis.gdal_enabled_drivers

git-svn-id: http://svn.osgeo.org/postgis/trunk@12475 b70326c6-7e19-0410-871a-916f4a2858ee