sd-journal: rework generic_array_bisect()

sd-journal: rework generic_array_bisect()

- Rename generic_array_bisect_one() -> generic_array_bisect_step(), as there
  is also generic_array_bisect_plus_one(), so the original name is confusing.
- Make generic_array_bisect_step() return TEST_GOTO_NEXT or TEST_GOTO_PREVIOUS
  when the current array does not contain any matching entries.
- Make generic_array_bisect_step() symmetric with respect to the direction
  we are going to, except for the journal corruption handling.
- Make generic_array_bisect_step() gracefully handle journal corruptions,
  so the corruption handling in the caller side can be mostly dropped.
- Especially, when the last entry in an array is corrupted, previously
  we tried to find a valid entry sequentially from the end of the array,
  but now we anyway bisect the array. That should improve performance of
  reading corrupted journal files.
- Return earlier when no entry linked to the chained array (n == 0).
- Add many comments.

No behavior change unless journal is corrupted.