(scroll-down-mark): Handle pc-select-override-scroll-error,
(scroll-down-nomark): Likewise.
(scroll-up-mark, scroll-up-nomark): Likewise.
(pc-selection-mode): Get rid of the advice on scroll-up, scroll-down.
(pc-selection-mode): Big doc fix. Don't bind f1.
(forward-line-mark): New function.
(forward-line-nomark): New function.
(backward-line-mark): New function.
(backward-line-nomark): New function.
(pc-selection-mode): Add keybindings for those functions.
(pc-select-override-scroll-error): New variable.