top: a few tweaks for those scrolling (mostly) changes

top: a few tweaks for those scrolling (mostly) changes

This patch just addresses some edge cases with respect
to 'unseen' tasks. Given the ability to preserve other
filters in the rcfile, it's entirely possible the very
first task(s) may not be visible at top startup. Also,
when switching between windows ('a'/'w') we should try
to always position its row #1 on some visible process.

Lastly, a window might have *NO* visible tasks at all.
Therefore, protect 'window_hlp' from an infinite loop.

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