char: expect ascii character names in \x or \(xy

Now neatroff expects x and xy to be ASCII characters in \x or \(xy,
\nx or \n(xy, and \*x or \*(xy.  UTF-8 character or register names can
be accessed with \[xyz], \n[xyz], or \*[xyz].
hyph: handle special characters

Before this change hyphenation was limited to utf-8 characters;
now it handles special characters (\x, \(xy, \[xyz]) as well.
Suggested by Carsten Kunze <carsten.kunze@arcor.de>.