cmd_{previous,next}_line used COLS and LINES, this adapts them to body_{cols,lines}. While there, also add the ability to scroll when trying to go past the beginning/end of the screen.

clminibufev_set was never set, so it would enqueue the event again...

in raw mode, C-s, C-q, C-c etc are passed as-is, while in cooked mode (the one cbreak gives us) they retain their usual meaning.

don't print spaces past the end of the window