busybox: disable PREFER_IPV4_ADDRESS

busybox: disable PREFER_IPV4_ADDRESS

PREFER_IPV4_ADDRESS is broken on IPv6-only hosts, as it causes busybox
utilities (ping, traceroute, ntpd) to forcibly use the A record instead of
the AAAA record when resolving a DNS name.  This obviously fails when
there is no IPv4 connectivity.  Since IPv6-only hosts or routers will only
become more common over time, disable PREFER_IPV4_ADDRESS to support this
use-case.

As a side-effect, disabling PREFER_IPV4_ADDRESS changes the default
resolution behaviour of busybox utilities on dual-stack hosts.  Busybox
utilities now simply use the order given by getaddrinfo(), so they will
now prefer IPv6 addresses when resolving a name with both A and AAAA
records if there is IPv6 connectivity.  This is in line with RFC 6724.

PREFER_IPV4_ADDRESS was likely intended to work around naive
implementations of getaddrinfo() that could return AAAA records first,
even on an IPv4-only host.  But both musl (since 1.1.3) and glibc
correctly implement RFC 6724 for getaddrinfo() and check connectivity to
determine the correct order in which to return records.  On IPv4-only
hosts, getaddrinfo() will return A records first, so there is no need for
the PREFER_IPV4_ADDRESS hack.

See also: https://bugs.busybox.net/show_bug.cgi?id=12381

Fixes: FS#84
Fixes: FS#2608
References: https://github.com/openwrt/openwrt/pull/4167
Signed-off-by: Alexander Traud <pabstraud@compuserve.com>
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>