More secure path mangling

[marcjansen]

This is a follow up from the excellent #280. I think the security might even be improved by the following PR.

I'd really like a review from @anikaweinmann or @mmacata on this; this is open for discussion.

[marcjansen]

Hmm. Probably some occasions missed. Hopefully just that.

[marcjansen]

Please have another look @anikaweinmann I marked this as ready for review now. I added some arguments to the two lines you remarked on. I'd love to hear your thoughts again.

[anikaweinmann]

@marcjansen do you want to add a r/w parameter to the functions? Otherwise it is also fine for me to merge it as it is.