Create a way to turn off labeling (or feature rendering) globally.

[sdlime]

Layered client interfaces often want just the labels for a map (or just the underlying features) so it would be handy to have a method to do this at the map level. Something like:

LABELMODE ALL|NONE|ONLY

Where ALL would get you all labels and underlying features (what we do now). NONE would get you just the feature rendering for a given map and ONLY would get you just the labels. This would affect primarily mapdraw.c where we'd do one check before actually rendering a label or a feature.

Steve