fix: reject non-directory mkdirat conflicts

[immanuwell]

Small fix for fileutil, no drama. MkdirAtExist used to swallow EEXIST even when the existing path was a regular file. That is a bit sus because callers can keep going as if a dir exists.

Repro:

1. create a regular file at the path MkdirAllAtExist is about to create
2. run cd common && go test ./fileutil -run TestMkdirAllAtExist/'existing_file_path_-_should_error' -v
3. before this patch it fails with Expected error when path exists as a regular file

Now EEXIST is ignored only when fstatat says the existing path is a directory. Existing dirs still work as before.

Checked:

• cd common && go test ./fileutil
• cd common && go vet ./fileutil
• make test-common