Improve 2.5D support for OGR and PostGIS input and GML output

When USE_POINT_Z_M is defined, this pull request adds support for 2.5D geometries in OGR input and PostGIS input.
For PostGIS, whe USE_POINT_Z_M is defined, this will result in the processing option FORCE2D to default to OFF.
It also adds support for outputing 2.5D geometries in GML output (WFS typically), provided that the "gml_geomtype" metadata item (layer level) is defined to Point25D, LineString25D, etc... (same syntax as for OGR 2.5D output : http://mapserver.org/output/ogr_output.html#geometry-types-supported. Which should by the way be improved to mention that 25D suffix !)