MINOR: applet: Use an option to disable zero-copy forwarding for all applets

MINOR: applet: Use an option to disable zero-copy forwarding for all applets

At the beginning of the 3.0-dev cycle, the zero-copy forwarding support was
added only for the cache applet with an option to disable it. This was a
hack, waiting for a better integration with applets. It is now possible to
implement the zero-copy forwarding for any applets. So the specific option
for the cache applet was renamed to be used for all applets. And this option
is now also checked for the stats applet.

Concretely, 'tune.cache.zero-copy-forwarding' was renamed to
'tune.applet.zero-copy-forwarding'.