Don't let the 'B' interactive spec default to buffers viewed in

Don't let the 'B' interactive spec default to buffers viewed in
	any visible frame.
	* buffer.c (Fother_buffer): Pass Qvisible as the second argument
	to Fget_buffer_window.
	* window.c (Qvisible): New symbol.
	(window_loop): In GET_BUFFER_WINDOW case, if frames is Qvisible,
	ignore non-visible frames.
	(Fget_buffer_window): Expand doc string to mention that Qvisible
	is an interesting value for FRAME argument.
	(syms_of_window): Init and staticpro Qvisible.
	* lisp.h (Qvisible): New declaration.