foamToVTK cannot write faceZones only

As noted by @Prashant - used to be able to hack foamToVTK with -noInternal and -excludePatches to get faceZones only, but now these are linked to the internal mesh (so that -no-internal really doesn't generate internal data fields).

Agreed that it probably makes more sense to explicitly request faceZones by name or regex as per foamToEnsight. This makes it more flexible and more consistent with foamToEnsight.