net: make Dial("tcp", ln.Addr().String()) work even with bad IPv6 config

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/45088
Reviewed-by: Paul Marks <pmarks@google.com>

src/net/dial_test.go

@@ -887,3 +887,21 @@ func TestCancelAfterDial(t *testing.T) {
 		try()
 	}
 }
+
+// Issue 18806: it should always be possible to net.Dial a
+// net.Listener().Addr().String when the listen address was ":n", even
+// if the machine has halfway configured IPv6 such that it can bind on
+// "::" not connect back to that same address.
+func TestDialListenerAddr(t *testing.T) {
+	ln, err := Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+	addr := ln.Addr().String()
+	c, err := Dial("tcp", addr)
+	if err != nil {
+		t.Fatalf("for addr %q, dial error: %v", addr, err)
+	}
+	c.Close()
+}