need to use return value of recvmsg for length of received data

asyncutils.nim

@@ -138,7 +138,9 @@ proc asyncRecvMsg*(fd: AsyncFD,
         retFuture.fail(newException(ValueError, "unexpected ancillary data"))
         return
 
-    dataBuffer.setLen(msg.msg_iov.iov_len)
+    # FIXME: This will not work on FreeBSD as recvmsg(2) says:
+    # recvmmsg() returns the number of messages received
+    dataBuffer.setLen(res)
     retFuture.complete((dataBuffer, ancillaryData))
 
   addRead(fd, cb)