Returns the buffer-name to use for help output.
Change all the code to use it instead of hard-coding *Help*.
(help-mode-map): Put back the mouse-2 binding.
(help-xref-stack): Change the format and make buffer-local.
(help-xref-stack-item): Make buffer-local.
(help-setup-xref): Do the `push' here rather than in help-do-xref.
(help-xref-interned): Display the face doc as well.
(help-follow-mouse): Re-introduce.
(help-xref-go-back): Adapt to the new use of help-xref-stack.
(help-do-xref): Don't `push' any more.
(help-follow): Also follow face names.
