Improving the extensibility for req/rep

This is an improvement on https://github.com/nanomsg/nanomsg/pull/264, marking static members as non-static so they can be referenced outside req.c and rep.c

In addition, methods nn_req_csend and nn_req_crecv seem to have been added, that perform the old functions of nn_req_send and nn_req_recv respectively.  Exposing these methods for extensibility.

Signed-off-by: Drew Crawford <drew@sealedabstract.com>