(usage): Clarify whether text or binary is the default.

(usage): Clarify whether text or binary is the default.
(split_3, main): BINARY is now a 3-way value.  All uses changed.
(digest_file): Likewise.  Clear *BINARY if we determine the file
to be text.  All uses changed.
(main): Don't report a file to be binary if we actually read it
as text in MS-DOS, because it was a terminal.