Should continue if process has no file handles left
Fixed response to file handle exhaustion

- libzmq no longer asserts when file handles are exhausted
- so all classes that create sockets have to be robust against this
- they return NULL on construction
Fixed zproxy to deal with socket exhausted errors

- also fixed zproxy_bind to return 0 on OK, -1 on errors
Merge pull request #293 from hintjens/master

Fixed response to file handle exhaustion