chase: reuse "done" to open fd of starting point

chase: reuse "done" to open fd of starting point

For readability that 'done' and 'fd' are always consistent with each other.

- dir_fd == AT_FDCWD:
  - path is absolute:
    - previous: fd = open("/")
    - current:  fd = openat(AT_FDCWD, "/")
  - path is relative:
    - previous: fd = openat(AT_FDCWD, ".")
    - current:  fd = openat(AT_FDCWD, ".")
- dir_fd >= 0:
  - dir_fd points to "/":
    - previous: fd = openat(dir_fd, ".")
    - current:  fd = openat(dir_fd, "/")
  - dir_fd does not point to "/":
    - previous: fd = openat(dir_fd, ".")
    - current:  fd = openat(dir_fd, ".")

Hence, this should not change any behavior. Just refactoring.