Polygon and Multipolygon distinguished by field type constraint - call for a generic AnyPoly type

I am looking for a way of creating a geometry field that has a
constrained type identical to the Shapefile spec.
Basically, Polygons and MultiPolygons are equals. Linestrings and
Multilinestrings are equals.
Is there a way to accomplish this with PostGIS?
If I make the field Polygon, then I can't insert Multipolygons, and vice versa.

Thanks,
Ben