When selecting ranges of byte offsets (as opposed to ranges of fields)

When selecting ranges of byte offsets (as opposed to ranges of fields)
and when --output-delimiter=STRING is specified, output STRING between
ranges of selected bytes.

(RANGE_START_SENTINEL): Define.
(output_delimiter_specified): New global.
(print_kth): Add parameter.  Adjust all callers.
(set_fields): Mark each range-start index with RANGE_START_SENTINEL.
(cut_bytes): When requested, output STRING between ranges of
selected bytes.
(main): Make a diagnostic a little clearer.
Based on a patch from Jan Nieuwenhuizen.