when isc_nm_listenstreamdns() is called with a local port of 0,
a random port is chosen. call uv_getsockname() to determine what
the port is as soon as the socket is bound, and add a function
isc_nmsocket_getaddr() to retrieve it, so that the caller can
connect to the listening socket. this will be used in cases
where the same process is acting as both client and server.
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