MINOR: sink: merge sink_forward_io_handler() with sink_forward_oc_io_handler()

MINOR: sink: merge sink_forward_io_handler() with sink_forward_oc_io_handler()

Now that sink_forward_oc_io_handler() and sink_forward_io_handler() were
unified again thanks to the previous commit, let's take a chance to merge
code that is common to both functions in order to ease code maintenance.

Let's add _sink_forward_io_handler() internal function which takes the
applet and a message handler as argument: sink_forward_io_handler() and
sink_forward_oc_io_handler() leverage this internal function by passing
the correct message handler for the desired format.