* Get the listening addresses of a listener.
* A listener can have more than one address for several reasons:
* - DNS host records may indicate more than one address
* - On a multi-homed host, listening on the default host "" will listen on all local addresses.
* - Some IPv4/IPV6 configurations may expand a single address into a v4/v6 pair.
* @param l points to the listener
* @param addrs an array of `pn_netaddr_t*` to be filled with pointers the listening addresses
* Pointers are invalid after the listener closes (PN_LISTENER_CLOSE event is handled)
* @param len is the length of the array @p addrs
* @return The actual number of addresses used for listening. This may be:
* - 0: addresses are unavailable.
* - <= @p len: all addresses were copied to @p addrs, remaining entries were set to NULL.
* - > @p len: the first @p len addresses were copied to @p addr.
* `pn_netaddr_listen(l, NULL, 0)` returns the number of listening addresses.
PNP_EXTERN const size_t pn_netaddr_listen(pn_listener_t *l, pn_netaddr_t** addrs, size_t len);