A couple of fixes to make the kioslave work again.

I had removed extern "C" at some point, over-eager cleaning... it is needed for klauncher to find the symbol.
Also, building with libgit2 with thread support requires a call to initialize thread local storage before any other calls of the api.
Fix opening of symlinks in bup kioslave.

Learning some more about kio... It does not copy symlinks, it just takes the target info from the stat() and then creates a new symlink on destination using that. But it currently only does that if src and dst are the same kioslave. Nevermind that for now, not a big problem. But it anyway, the intention is that calling get() on a symlink should download the target file. So we do that now.