Problem: ZMQ_USEFD does not follow conventions

Solution: rename socket option (and variables and files) from usefd
to use_fd.

src/options.hpp

@@ -235,7 +235,7 @@ namespace zmq
         //  When creating a new ZMQ socket, if this option is set the value
         //  will be used as the File Descriptor instead of allocating a new
         //  one via the socket () system call.
-        int usefd;
+        int use_fd;
     };
 }