Here's an updated patch:
- address returned by recvfrom()
- recv flags (MSG_PEEK)
- congestion behavior

I've also added a bunch of constants:
- all the typical SO_ constants
- CMSG flags
- RDMA-related options (RDMA is probably one of the most useful features of RDS)
- I chose to let aside INFO flags, since they don't appear in the man pages  (and neither here: https://github.com/agrover/python-rds/blob/master/rdma.py, a ctypes-based RDS implementation I stumbled upon)
- I didn't add control messages tests: they all pertain to RDMA, which is rather tricky to test to test from Python (and my header files don't define them)