Reuse some of the old members.
The "input" and "output" function pointers are actually of type
xmlCharEncConvFunc, accepting an additional argument. For default
handlers, this argument is unused, so this should work with most ABIs.
For iconv handlers, these function pointers used to be NULL but now
point to a function which requires the extra argument.
"iconv_in" and "iconv_out" are made void pointers. "uconv_in" and
"uconv_out" are renamed and made void pointers. This is unlikely to
cause issues.
We now expect that the built-in conversion functions correctly report
XML_ENC_ERR_SPACE. For UTF8ToHtml and the ISO-8859-X code, this will be
done in the following commits.
282ec1d548