rzmq::receive.socket() honors the ZMQ_DONTWAIT flag

Here's the patch. Works for me on linux amd64. Please test on your box to see if I've missed anything.

I elided the #define NDEBUG at the top of interface.cpp  because the compiler was warning me that it was redundant during package installation.

Jason
Question -- I'm not actually sure what this is trying to say? (How does receive.null.msg interact?)