* cppfiles.c (open_file): Fail directories silently, but
	with an errno of NOENT set.
	(read_include_file): Move the common exit code to the sole
	caller.  Return an int indicating success or failure.  Let
	open_file handle directories.
	(stack_include_file): If read_include_file fails,
	push a "null" buffer.

From-SVN: r41138