(Info-select-node): Turn header line on or off here.

(Info-select-node): Turn header line on or off here.
(Info-setup-header-line): Function deleted.
(Info-mode): Make Info-header-line local.
(Info-fontify-node): Ignore Info-use-header-line.
Unconditionally compute what the header line should be, and
store it on the `header-line' property of the node's first char.
Bind [header-line down-mouse-1].
Use `keymap' property, not `local-map'.
Bind [mouse-2] for mouse clicks on the node line in the text.