texinfo.tex: non-ASCII support completely broken for XeTeX and LuaTeX

[Werner LEMBERG]

[texinfo.tex 2022-12-19.22]


Consider this input file

```
\input texinfo
·
@bye
```

(the character is U+00B7, MIDDLE DOT).  Only pdfTeX produces this
character; both XeTeX and LuaTeX silently discard it.

If you replace '·' with, say, 'Æ' (U+00C6, LATIN CAPITAL LETTER AE),
both XeTeX and LuaTeX abort with

```
Runaway argument?
! Paragraph ended before @UTFviiiTwoOctets was complete.
<to be read again> 
@par 
```

If you replace it with 'ÿ' (U+00FF, LATIN SMALL LETTER Y WITH
DIAERESIS), you get

```
! Undefined control sequence.
l.3 ÿ
```

If you replace it with 'Ā' (U+0100, LATIN CAPITEL LETTER A MACRON),
you get a~log warning

```
Missing character: There is no Ā ("100) in font cmr10!
```

inspite of a proper `\DeclareUnicodeCharacter` line for this character
in `texinfo.tex`.

Ouch, ouch, ouch :-)


    Werner