Fix C based tagtransform for non-hstore mode

Reported in github issue #35

The previous fix b80cb3ebd5cdee1384f48b8f571704bf82187d16 did not fully fix everything.
(hopefully this time it will)

There are still some differences (with respect to tag copying from member ways of
multipolygon relations) to the original code, but they should all be either improvements,
or the correct behaviour is ambivalent.