MINOR: resolvers: add "dns-accept-family auto" to rely on detected IPv6

MINOR: resolvers: add "dns-accept-family auto" to rely on detected IPv6

Instead of always having to force IPv4 or IPv6, let's now also offer
"auto" which will only enable IPv6 if the system has a default gateway
for it. This means that properly configured dual-stack systems will
default to "ipv4,ipv6" while those lacking a gateway will only use
"ipv4". Note that no real connectivity test is performed, so firewalled
systems may still get it wrong and might prefer to rely on a manual
"ipv4" assignment.