ipn/ipnlocal, tailcfg: add MagicDNS opt-in attr for IPv6 AAAA records

Until we turn on AAAA by default (which might make some people rely on
Happy Eyeballs for targets without IPv6), this lets people turn it on
explicitly if they want.

We still should add a peer cap as well in the future to let a peer
explicitly say that it's cool with IPv6.

Related: #9574

Updates #1813
Updates #1152

Change-Id: Iec6ec9b4b5db7a4dc700ecdf4a11146cc5303989
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -5056,6 +5056,8 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.
 		!nm.GetAddresses().ContainsFunc(tsaddr.PrefixIs4)
 	dcfg.OnlyIPv6 = selfV6Only
 
+	wantAAAA := nm.AllCaps.Contains(tailcfg.NodeAttrMagicDNSPeerAAAA)
+
 	// Populate MagicDNS records. We do this unconditionally so that
 	// quad-100 can always respond to MagicDNS queries, even if the OS
 	// isn't configured to make MagicDNS resolution truly
@@ -5092,7 +5094,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, peers map[tailcfg.NodeID]tailcfg.
 			// https://github.com/tailscale/tailscale/issues/1152
 			// tracks adding the right capability reporting to
 			// enable AAAA in MagicDNS.
-			if addr.Addr().Is6() && have4 {
+			if addr.Addr().Is6() && have4 && !wantAAAA {
 				continue
 			}
 			ips = append(ips, addr.Addr())