True if Atom is a valid Turtle PN_LOCAL name. The
PN_LOCAL name is what can follow the : in a resource. In the new Turtle,
this can be anything and this function becomes meaningless. In the old
turtle, PN_LOCAL is defined similar (but not equal) to an XML name. This
predicate is used by rdf_save_turtle/2
to write files such that can be read by old parsers.