dns: fail if ipv6 is disabled and resolv.conf has only v6 nameserves

if resolv.conf lists no nameservers at all, the default of 127.0.0.1
is used. however, another "no nameservers" case arises where the
system has ipv6 support disabled/configured-out and resolv.conf only
contains v6 nameservers. this caused the resolver to repeat socket
operations that will necessarily fail (sending to one or more
wrong-family addresses) while waiting for a timeout.

it would be contrary to configured intent to query 127.0.0.1 in this
case, but the current behavior is not conducive to diagnosing the
configuration problem. instead, fail immediately with EAI_SYSTEM and
errno==EAFNOSUPPORT so that the configuration error is reportable.