Add optional meta-information to version-3 format.

* Makefile (ZFLAGS): Add a comment about how to enable meta-info.
* tzfile.5: Describe meta-information.
* zic.8: Document new options -n and -o, which cause zic to
generate meta-info.
* zic.c: Include <stddef.h>, for ptrdiff_t.
(genoption, genoptions, genname): New static vars.
(usage): Summarize new options.
(addgenoption, writevalue): New function.
(main, writezone): Add support for new options.