Make macro xhtml export more secure

Actually I wanted to do that in cc87f8100 but forgot to adjust the original
solution completely. Now we do not search for an arbitrary latexkeys instance
which just happens to have the same name as the macro, but we only use the
symbol that was explicitly set for global macros.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.12.30-13.00