add ud_pingpong.c

Would like to check in first iteration of ud_pingpong.  Needs some work for specifying desired local address, but usnic provider does run this successfully.

This one is intended to be mostly just a clone of pingpong.c.  I have another version of a ud_pingpong that gives quite a bit lower latency I will submit soon, intending that both coexist in the repo as different illustrations of how to code with libfabric.