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.