top: force return to row 1 for thread mode transitions

top: force return to row 1 for thread mode transitions

This program does a good job of policing that vertical
scrolled position, ensuring that total tasks are never
exceeded. However, during transitions from thread mode
to normal task mode (the 'H' toggle) that wasn't true.

And while there was no real harm done, it did make the
use of up/down arrow keys "appear" disabled especially
if that scroll message was not displayed ('C' toggle).

This patch simply forces a return to row #1 whenever a
user toggles that display between thread & task modes.

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