MEDIUM: applet: Add support for zero-copy forwarding from an applet

MEDIUM: applet: Add support for zero-copy forwarding from an applet

Thanks to this patch, it is possible to an applet to directly send data to
the opposite endpoint. To do so, it must implement <fastfwd> appctx callback
function and set SE_FL_MAY_FASTFWD flag.

Everything will be handled by appctx_fastfwd() function. The applet is only
responsible to transfer data. If it sets <to_forward> value, it is used to
limit the amount of data to forward.