Some machines can be configured (or came/come configured) in such a
state that IPv6 only half works: you can bind on [::]:n but not
connect back to it.

This implements a fallback such that it's guaranteed that this pattern
works:

   ln, err := Listen("tcp", ":0")
   ...
   addr := ln.Addr().String() // "[::]:n"
   c, err := Dial("tcp", addr)

... which is also now tested. It will first try to dial "[::]:n", as
before, but if that dial fails, it will also try "0.0.0.0:n".

Fixes #18806 (contains more details)
Fixes #20611 (I was going to fix nacl later, but it was easy enough)

Change-Id: I1107eb197e902ae8185c781ad1bc4e2bc61d1f4c
Reviewed-on: https://go-review.googlesource.com/45088Reviewed-by: Paul Marks <pmarks@google.com>