Using the getaddrinfo order is only okay if we
are smart enough to try multiple addresses in Dial.
Since the code does not do that, we must make
the right first choice, regardless of what getaddrinfo
does, and more often that not that means using the
IPv4 address, even on IPv6 systems.  With the CL
applied, gotest fails in package net on OS X.

helix.cam=; gotest
...
--- FAIL: net.TestDialGoogleIPv4 (1.05 seconds)
        -- 74.125.226.179:80 --
        -- www.google.com:80 --
        Dial("tcp", "", "www.google.com:80") = _, dial tcp [2001:4860:800f::69]:80: address family not supported by protocol family
        -- 74.125.226.179:http --
        -- www.google.com:http --
        Dial("tcp", "", "www.google.com:http") = _, dial tcp [2001:4860:800f::69]:80: address family not supported by protocol family
        -- 074.125.226.179:0080 --
        -- [::ffff:74.125.226.179]:80 --
        -- [::ffff:4a7d:e2b3]:80 --
        -- [0:0:0:0:0000:ffff:74.125.226.179]:80 --
        -- [0:0:0:0:000000:ffff:74.125.226.179]:80 --
        -- [0:0:0:0:0:ffff::74.125.226.179]:80 --
FAIL
gotest: "./6.out" failed: exit status 1

««« original CL description
net: name-based destination address selection

getaddrinfo() orders the addresses according to RFC 3484.

This means when IPv6 is working on a host we get results like:
    []string = {"2001:4810::110", "66.117.47.214"}

and when it's not working we get:
    []string = {"66.117.47.214", "2001:4810::110"}

thus can drop firstFavoriteAddr.

This also means /etc/gai.conf works on relevant systems.

R=rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/4557058

»»»

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4532101