Fix two issues around help-window-select.  (Bug#11039) (Bug#19012)

* help.el (help-window-old-frame): New variable.
(help-window-select): Default to nil (Bug#11039).  Rewrite
doc-string.
(help-window-setup): When the help window appears on another
frame and `help-window-select' is non-nil, give that frame input
focus too (Bug#19012).
(with-help-window): Store selected frame in
help-window-old-frame.