Improve PostScript output.

Some highlights in no particular order:

%destructor was somehow lost from the list that follows, it should be
part of it.

Use .Ic for yacc directives when they are defined, .Ql otherwise.

Use explicit .Sq Li (instead of .Ql) in description of %destructor to
make sure the result is consistently quoted.  It more readable that
way.

Use .Va and .Vt where appropriate.

C preprocessor directives are marked up with .No (a nop), so that it's
easy to switch them to something else if need be.  For now just use
them as plain words.

(uwe)