Return the DNS PTR record for player 'ob's IP . An asynchronous DNS
reverse lookup is triggered on new connection to the server.
Before lookup finishes, this function returns same result as
After lookup finishes, if lookup is succesful, this funciton will
return the DNS PTR value for the IP of this object. If lookup failed,
(due to network issues, PTR record not configured for this IP etc),
this function will continue to return same result as 'query_ip_num‐
The result is cached, there is no overhead for this function.