rewrite all TFRewrite instances to use the new pkgTagSection::Write

While it is mostly busywork to rewrite all instances it actually fixes
bugs as the data storage used by the new method is std::string rather
than a char*, the later mostly created by c_str() from a std::string
which the caller has to ensure keeps in scope – something apt-ftparchive
actually didn't ensure and relied on copy-on-write behavior instead
which c++11 forbids and hence the new default gcc abi doesn't use it.