Tolerate EEXIST in make_ancestors()

[rdjjke]

Now, the file_create function (used to create destination placeholders for bind-mounts) doesn't fail if any target file already exists, which can happen if nvidia-container-cli configure runs concurrently for the same path. However, the creation of the parent directories is not equally race-tolerant.

make_ancestors checks whether each directory exists before calling mkdir. If another process creates the directory in the window between that check and the mkdir, the mkdir fails with EEXIST and aborts the whole nvidia-container-cli configure.

This PR contains a small patch to tolerate EEXIST in make_ancestors, matching the idiom already used for the final mkdir/symlink in file_create.