Check for EWOULDBLOCK as well as EAGAIN.

- POSIX allows recv to return either EAGAIN or EWOULDBLOCK, and
  allows them to be the same value. On nearly every modern POSIX
  system, it returns EAGAIN, and they are the same value. But
  Winsock returns EWOULDBLOCK, and defines them as different
  values. So, we need to check both, not just EAGAIN.
Merge pull request #6 from abarnert/ewouldblock

Check for EWOULDBLOCK as well as EAGAIN.