Allow disabling geometry output for WFS

[sdlime]

In some cases one might not want to output the geometry associated with a
feature as part of a WFS response. I propose taking advantage of the
gml_geometries metadata element. That element allows you to define the
geometry(ies) associated with a layer (MapServer will create a default if not
set). By setting that value to "none" that would trigger MapServer to:
  
  - ommit the geometry from the describeFeatureType response
  - ommit the geometry (and bounding box) from a WFS query response

I've tested the proposed solution and it works nicely and could be taken
advantage of via WFS filters that exclude certain properties by setting metadata
on-the-fly.