prevent accidental amalgamation of graphic token chars in output

prevent accidental amalgamation of graphic token chars in output

This addresses #2713. Many thanks to @notoria for reporting this
excellent case!

Example:

    ?- portray_clause(A = @).
    A= @ .
       true.

At other positions the now inserted space is unnecessary, as in:

    ?- portray_clause((head:- @,b)).
    head :-
       @ ,
       b.
       true.

The toplevel has a similar issue:

    ?- C = # ; false.
       C = # |<-- cursor is here; redundant space after #

There may be a way to solve this issue for all cases like this.