path: Improve $PATH search directory case

Previously:

1. last_error wouldn't be updated with errors from is_dir;
2. We'd always issue a stat(), even for binaries without execute;
3. We used stat() instead of access(), which is cheaper.

This change avoids all of those, by only checking inside X_OK-positive
case whether access() works on the path with an extra slash appended.
Thanks to Lennart for the suggestion.