Center point in selected window and maybe redisplay frame.
With prefix argument ARG, recenter putting point on screen line ARG
relative to the selected window. If ARG is negative, it counts up from the
bottom of the window. (ARG should be less than the height of the window.)
If ARG is omitted or nil, then recenter with point on the middle line of
the selected window; if the variable `recenter-redisplay' is non-nil,
also erase the entire frame and redraw it (when `auto-resize-tool-bars'
is set to `grow-only', this resets the tool-bar's height to the minimum
height needed); if `recenter-redisplay' has the special value `tty',
then only tty frames are redrawn.
Just C-u as prefix means put point in the center of the window
and redisplay normally--don't erase and redraw the frame.
(fn &optional ARG)