join: Add the -z, --zero-terminated option

join: Add the -z, --zero-terminated option

* NEWS: Mention join's new option: --zero-terminated (-z).
* src/join.c: Add new option, --zero-terminated (-z), to make
join use the NUL byte as separator/delimiter rather than newline.
(get_line): Use readlinebuffer_delim in place of readlinebuffer.
(main): Handle the new option.
(usage): Describe new option the same way sort does.
* doc/coreutils.texi (join invocation): Describe the new option.
* tests/misc/join.pl: add tests for -z option.