MINOR: resolvers: add command-line argument -4 to force IPv4-only DNS

MINOR: resolvers: add command-line argument -4 to force IPv4-only DNS

In order to ease troubleshooting and testing, the new "-4" command line
argument enforces queries and processing of "A" DNS records only, i.e.
those representing IPv4 addresses. This can be useful when a host lack
end-to-end dual-stack connectivity. This overrides the global
"dns-accept-family" directive and is equivalent to value "ipv4".