[Groff] \o and \z do not work for -Tutf8

[Bernd Warken]

For `gpinyin', the European-like Chinese, I need to create character
overstrikes for the 1st (a macron) and the 3rd tone (a caron).
To append an accent over the ü and Ü characters (u/U umlaut) for all
4 tones seems to be even more complex.

The overstrike with `\o' and `\z' preduce on `-Tpdf', etc., a plus
sign within the digit 0 for both escapes.  But both escapes do not
work on `-Tutf8' or `nroff': 

$ echo "A\o'0+'\z0+Z" | nroff
A++Z

which is wrong.

Both escapes are documented in Osanna/Kernioghan `CSTR 54' and
many other ancient `roff' documentations.  So `nroff' should be able
to do that as well.  Every typewriter could do such overstriking.

I had also trouble with `\h' and `\v', but for that later on.

Bernd Warken