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/cp.c (do_copy): Use it.

	* src/cp.c (do_copy): Don't assume argc is positive.
	Don't bother to lstat dest, since copy() will do that for us.
	Use "const" to avoid the need for cast.

	* 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/cp.c (do_copy): Remove test for trailing slash, since
	target_directory_operand now does this.

	* src/cp.c (main): Reject multiple target directories.
	Check whether a specified target is a directory when parsing the
	options, using stat.  This gives more-accurate diagnostics.