Use more-consistent rules among cp, ln, and mv when dealing with

	Use more-consistent rules among cp, ln, and mv when dealing with
	last operands that are (or look like) directories.

	* src/cp.c (target_directory_operand): New, nearly-common function,
	It reports an error if the destination appears to be a directory
	(e.g., because it has a trailing slash) but is not.
	* src/ln.c, src/mv.c: Likewise.
	* src/cp.c (do_copy): Use it.
	* src/ln.c (main): Likewise.
	* src/mv.c (main): Likewise.

	* src/cp.c (do_copy): Don't output a usage message because of file
	problems (e.g., an operand is not a directory).  Use it only for
	syntax.  Standardize on "target %s is not a directory" for the
	diagnostic.
	* src/ln.c (main): Likewise.
	* src/mv.c (main): Likewise.

	* src/cp.c (do_copy): Remove test for trailing slash, since
	target_directory_operand now does this.
	* src/ln.c (main): Likewise.
	* src/mv.c (movefile): Likewise.

	* src/ln.c (isdir): Remove decl; no longer needed.
	* src/mv.c (isdir, lstat): Likewise.

	* src/ln.c (main): Use int to count to argc, not unsigned int.
	This handles negative operand counts.
	* src/mv.c (main): Likewise.

	* src/mv.c (do_move): Don't call hash_init; expect the caller to
	do it, for consistency with cp.c and ln.c.  All callers changed.
	(movefile): dest_is_dir parameter is now bool, not int.
	(main): Standardize on "missing destination file operand after %s"
	for the diagnostic, for consistency with cp.c.