Examples:
?- portray_clause(((+) :- a=b,(-))).
(+) :-
a=b,
(-).
true.
?- portray_clause((a :- (b :- c))).
a :-
(b:-c).
true.
This addresses #2715.
Currently, more round brackets than needed are emitted. Ideally, a
better approach to solve the issue here and also in toplevel.pl is
found in the future.