Some changes based on feedback.

* fth/file.fth: Clearer license statement.
(\N, \R, SKIP-\N): Upcase properly.
(READ-LINE): Use 0<> instead of 0< for the (unlikely) case that
we have large unsigned numbers with the highest bit set.
Also, skip-\n can overwrite the \r that we just detected.

* fth/t_file.fth: Add tests with buffers that are shorter and exaclty
as long as the line.