use round brackets for goals involving operators

use round brackets for goals involving operators

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.