#5 Add callback for scrolling action

[Kevin Quick]

This patch allows registration of a callback that is
invoked whenever a scrolling operation occurs. This
patch requires patch #1438844 and enhances that patch
to call a registered callback function with the line
removed by the scroll (and which will be used to supply
the newly exposed line).

The most obvious use of this patch is to provide
scrollback history for the Rote window.

[Kevin Quick]

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Also attached boxshell2.c demo. This is an updated version
of boxshell.c that uses select instead of busywaiting, and
that also demonstrates the use of the scrolling callback.

[Kevin Quick]

boxshell2.c -- updated demo

[Kevin Quick]

scrolling callback patch (updated)