If we got IPv4 lookup response, we don't wait for IPv6 lookup
response. This is because DNS server may drop AAAA query and we have
to wait for the long time before timeout. We don't do the inverse,
because, based on todays deployment of DNS server, almost all of them
can respond A query just fine.
Currently, aria2 checks configured addresses at the startup. But there
are chances that interfaces are not setup at that moment. For example,
if aria2 is used as daemon, it may start before network interfaces
up. To workaround this, we check addresses again if both addresses are
not configured at the startup.
Issue A record query only when non-loopback IPv4 address is
configured. Likewise, issue AAA record query only when non-loopback
and non-linklocak IPv6 address is configured.