Clip the 'lines' parameter to scroll() at the size of the scroll

Clip the 'lines' parameter to scroll() at the size of the scroll
window. scroll() iterates that many times, so this prevents a tedious
wait if you give a very large parameter to ESC[L or ESC[M, for
example.

A side effect is that very large requests for upward scrolling in a
context that affects the scrollback will not actually wipe out the
whole scrollback: instead they push just the current lines of the
screen into the scrollback, and don't continue on to fill it up with
endless boring blank lines. I think this is likely to be more useful
in general, since it avoids wiping out lots of useful scrollback data
by mistake. I can imagine that people might have been using it
precisely _to_ wipe the scrollback in some situations, but if so then
they should use CSI 3 J instead.

[originally from svn r9677]