This reverts commit d1b78e8d.

The problem is that upon exiting the screen is likely to be cleared on
most terminals (not the one we embed in varnishtest for example) so it
doesn't give any chance to the user to read the output.

The consensus is to restore the previous behavior and fix the manual
instead.

Conflicts:
	bin/varnishtest/tests/u00004.vtc
	bin/varnishtop/varnishtop.c

Breaks:
	bin/varnishtest/tests/r02686.vtc

Refs #2721