currently the SCE-MI standard requires the Receive() callback
to be a non-NULL pointer to a function [1].  However, it may
be useful to allow this to be NULL with the meaning that any
messages from the output port are to be ignored.  The
infrastructure can further use this to ignore messages on
the hardware side.  This could be useful with ReplaceBinding()
to shut up an output port (`unbind' it) that is no longer needed
in the current session.

[1] See SCE-MI 1.0 p. 43 under the Receive() heading.  Here it
is stated that it shall be an error if Recveive() is NULL.

Also, this would allow us to define BindMessageOutPort() and
SceMiMessageOutPortProxy::ReplaceBinding() symmetrically with
the input port equivalents in regards to NULL binding
arguments.

Now, following John's line of thought, it can be argued that
shutting up a port is really an application layer thing.
However, without the above mechanism it is not possible to
write simple monitor transactors with only output ports that
can also be shut down in the middle of a session.  One would
have to include an input port (somewhere) to be able to tell
the port to activate or deactivate.

Per