Add -z option to uniq. Originally proposed by Egmont Koblinger.

Add -z option to uniq.  Originally proposed by Egmont Koblinger.
* NEWS: Mention uniq's new option: --zero-terminated (-z).
* src/uniq.c: Add new option, --zero-terminated (-z), to make
uniq use the NUL byte as separator/delimiter rather than newline.
(check_file): Add a parameter: delimiter.  Update caller.
Use readlinebuffer_delim in place of readlinebuffer everywhere.
(main): Handle the new option.
(usage): Describe new option the same way sort does.
* doc/coreutils.texi (uniq invocation): Describe the new option.