WIP: Check essential directories permissions at startup

Hey @maxking, we're not fully done yet. :p

(I'm sorry)

Here is the last changes I wanted to offer to fix the issue I described.

These are not required, but maybe it's worth a review, and if you have some ideas or suggestions I'd gladly take them into account.

On the current changes, there is one thing that is untested. I'll open a specific discussion on this line of code.