Improve tzfile format as suggested by Zefram and Alan Barrett.

* tzfile.5: Be a bit more careful about wording for integers.
Use binary terminators for the byte strings, not newlines.
Add a size field.  Terminate by another NUL.  Mention where
future extensions are expected to go.
* zic.c (writezone): Implement this.
(addgenoption): Omit namelen check; no longer needed.
(writevalue): Remove.