Using vut->last_sighup was not adequate, because the VUT_Main loop might
have terminated for some other signal or because of EOF.

On the other hand, we do not want to end the curses loop just because
the VUT_Main loop has ended in order to continue to display the last
state (and maybe the EOF in the top right corner).

So while I see that checking just one flag has some beauty to it, I do
think that directly checking the signal counters is both simple and
robust.

Fixes #3088 for varnishtop and varnishhist