True if URI has a scheme. The semantics is the same as the 
code below, but the implementation is more efficient as it does not need 
to parse the other components, nor needs to bind the scheme. The 
condition to demand a scheme of more than one character is added to 
avoid confusion with DOS path names.
uri_is_global(URI) :-
        uri_components(URI, Components),
        uri_data(scheme, Components, Scheme),
        nonvar(Scheme),
        atom_length(Scheme, Len),
        Len > 1.