top: normalize vertical scrolling for hidden processes

top: normalize vertical scrolling for hidden processes

To my knowledge, nobody has ever complained about some
anomalies when scrolling vertically if tasks should be
hidden from view. This can happen with the user filter
('u/U') or other filter ('o/O') features. And although
some tasks are not shown, they still impact scrolling.

This is most apparent when that scroll coordinates msg
is on ('C') & up/down arrow keys used (vs. pgup/pgdn).

Now that we can collapse/expand forked children, there
is a potential for yet more of those hidden processes.

So this commit normalizes vertical scrolling providing
an expected behavior. In other words, the up/down keys
skip the unseen tasks to reposition on a visible task.

Signed-off-by: Jim Warner <james.warner@comcast.net>