Limit the scope of some fields to some entity types

Spin off from #538164: Comment body as field
The 'X as field' trend creates a few fields like (node_)body, comment_body, block_body, etc., that only make sense to be 'shared' within a given entity type. Having comment_body show up in the UI for 'add existing field' will only clutter the 'list of existing field' selector and confuse people.

I think this calls for an (optional) $field['entity_types'] = array('comment') property, forbidding the creation of an instance on non-comment bundles (field_create_instance() raises an exception).