Seller/vendor fixes, towards https://github.com/rvguha/schemaorg/issues/88
See https://www.w3.org/wiki/images/7/7e/ProviderSellerVocabularyRe-DesignProposal.pdf
Draft implementation detail overview: https://www.w3.org/wiki/WebSchemas/SellerVendorCleanup#Implementation_Details

This needs checking and domains for 'broker' confirming. Also examples.txt will need updates.