return EEXIST instead of EROFS for directories in ~/config

[nephele]

Currently when trying to create a directory in ~/config that already exists EROFS is returned as error, it would be nice to return EEXIST instead.

While both errors are correct (and posix does not seem to specify which we should use) This leads to some ported programms having to be patched, as they will try to use XDG_CACHE_HOME for example, which we set as ~/config/cache currently.

When the programm tries to mkdir ~/config/cache most ported apps seem to treats status code 0 and EEXIST as sucess, and all else as failures.

The correct way for such ported apps in this instance would be to accept EROFS, and then do stat or something else to confirm the direcory exists.

I propose we change the behaviour here (of packagefs) to eliminate this edgecase.

[waddlesplash]

This is more due to the kernel, actually. packagefs doesn't even provide a create_dir hook, so the kernel returns EROFS for it.

What do Linux and the BSDs do here if you mount a partition read-only but union-mount another filesystem inside it? That's the equivalent case for packagefs.