ls: add --null option (Bug#49716)

ls: add --null option (Bug#49716)

* NEWS, doc/coreutils.texi (General output formatting):
* src/ls.c (usage): Document this.
* src/ls.c (NULL_OPTION): New constant.
(long_options): Add --null.
(eolbyte): New static var.
(dired_dump_obstack, main, print_dir, print_current_files)
(print_many_per_line, print_horizontal, print_with_separator):
Output eolbyte instead of '\n'.
(decode_switches): Decode --null.
* tests/ls/null-option.sh: New file.
* tests/local.mk (all_tests): Add it.