wgengine: return explicit lo0 for loopback addrs on sandboxed macOS

fixes tailscale/tailscale#TODO

The source address link selection on sandboxed macOS doesn't deal
with loopback addresses correctly.  This adds an explicit check to ensure
we return the loopback interface for loopback addresses instead of the
default empty interface.

Specifcially, this allows the dns resolver to route queries to a loopback
IP which is a common tactic for local DNS proxies.

wgengine/userspace.go

@@ -1580,6 +1580,11 @@ type fwdDNSLinkSelector struct {
 }
 
 func (ls fwdDNSLinkSelector) PickLink(ip netip.Addr) (linkName string) {
+	// sandboxed macOS needs some extra hand-holding for loopback addresses.
+	if ip.IsLoopback() && version.IsSandboxedMacOS() {
+		return "lo0"
+	}
+
 	if ls.ue.isDNSIPOverTailscale.Load()(ip) {
 		return ls.tunName
 	}