Write more explicit table header for phonetic spelling.

This reverts commit 223ff9c8ad.

Previous list was actually wrong. Previously it3 and it4 voices were configured,
but now there are entries for all four voices.

Z-SAMPA is a more complex specification than Kirshenbaum, X-SAMPA,
and CXS. Additionally, it specifies phonemes such as palatal and
velar trills that are not defined by IPA and in the context of the
velar trill is marked as being impossible.

As such, Z-SAMPA will not be supported in eSpeak NG for now. That
may change in the future once support for the other transcription
schemes has been implemented.

This makes it clearer that both variant and private-use tags are
used.

According to page 5 of [RFC5645](https://tools.ietf.org/html/bcp47#page-1-5),
variant subtags must have 5 to 8 characters. A 9-character subtag is
sytactically ill-formed and should be rejected by conforming parsers.

Technically, BCP47 does not allow the use of unregistered variant subtags.
For `fonkirsh`, I’ve filed a
[language variant subtag registration request](https://mailarchive.ietf.org/arch/msg/ietf-languages/1IrLl3n4wJ4Fr1xXV34QquJn8Bc)
with the IETF. If IETF rejects my request, we could probably add
Kirshenbaum to Unicode’s registry for [RFC6497](https://tools.ietf.org/html/rfc6497).

Not sure what to do about the other non-registered variants. If they’re
commonly used, it might be an option to register them as well, but
they seem to be even more exotic than Kirshenbaum, and we wouldn’t to
bloat the BCP47 registry too much. If espeak-ng really wants to handle these
alphabets, it would probably make sense to use BCP47 private-use extensions,
such as `x-fonzsamp`. According to IETF BCP47, private-use subtags can have
up to (but not more than) 8 characters, so I’m shortening these too.

This reverts commit 39994150ba.

This reverts commit 9689cc8860.