Don't set AI_ADDRCONFIG, even when it exists

Don't set AI_ADDRCONFIG, even when it exists

Basically, the issue described in #281 is that the test suite uses the
IPv4 address 127.0.0.1 on an IPv6-only host. But this is the "safest"
thing to do in (almost) all other setups: relaying on DNS host names
makes things even more complex, as different systems map 127.0.0.1
differently (including the reverse lookup; that's why we switched to
127.0.0.1 back in 2014, see commit 3f807e10457).

But with AI_ADDRCONFIG set, on an IPv6-only host, we prevent 127.0.0.1
to get translated properly, even when the loopback interface has this
address configured! So don't set it any more.

The drawback is that the resolver possibly returns more addresses now,
even of an unsupported/not connected address family; but this shouldn't
do much harm in practice, as ngIRCd iterates over all returned addresses
while trying to establish an outgoing connection.

Closes #281.