Sometimes the netsum server is blocked waiting for control messages
while it has been sent by the client.

It is because SO_RCVLOWAT is tuned to message size (ctrl->size). Since
the control message size is only 13 bytes, then recvfrom() blocks on
control message waiting for more bytes or a timeout.

This patch fixes the issue by letting SO_RCVLOWAT to its default value
(1). Since the {recv,send}_message functions are able to deal with
incomplete messages, then it should work just fine.