This is achieved by adding a new "message" field to the rp_elem struct
that can be set independent of the rp_elem "source". I also added a
special source called "MESSAGE" that implies there is no cptr contained
in an elem, only a message.

I also modified the CPInvokeRPSendArgs in the protocol (and CN_RP_SEND
stuff in dispatch) to support a cptr, a message or both.

CN_RP_RECV (both in the dispatch and in obj.c) has been modified to
support returning message, cptr, or both. This uses that new multi-item
response stuff added previously to get both the message and cptr to the
client if they are present. One part that temporarily tripped me up was
correctly generating a cn_dispatch_result_t for waiting recvs. I fixed
it by exposing the cn_dispatch_result_item generation stuff in
dispatch.c to obj.c.