nl: support multi-byte section delimiters

nl: support multi-byte section delimiters

* NEWS: Mention the improvement.
* src/nl.c: Include mcel.h.
(DEFAULT_SECTION_DELIMITERS): Resize to fit 2 multi-byte characters.
(section_del_len): New variable.
(check_section): Compare against section_del_len instead of 2.
(main): Support multi-byte characters for the -d option.
* tests/nl/multibyte.sh: New file.
* tests/nl/nl.sh: New file, moved from tests/misc/nl.sh.
* tests/local.mk (all_tests): Add the new test. Adjust the existing
tests file name.
* cfg.mk (exclude_file_name_regexp--sc_space_tab): Adjust Adjust the
existing tests file name.