Fix / workaround hang on server start

This changeset allows packet transmission as soon as the interface is plugged, but before listen() is called i.e. before the application is able to receive packets. This makes the library less likely to cause a deadlock when used by a TCP/IP stack, where configuring an interface (i.e. adding an IP before calling listen()) wants to transmit a gratuitous ARP.

@mor1 @avsm -- what do you think?