Before this commit, `CopyOnWriteFs` would return `syscall.EEXIST` in `Mkdir` and `MkdirAll` when a directory already exists.
The main problem with this is that `os.IsExist` returns `false` for that error on Windows.
These methods now return `os.ErrExist`, which is in line with how the other file systems behave.
Fixes#189
Bjørn Erik Pedersen