net/dns: relax systemd-resolved detection.

Reported on IRC: a resolv.conf that contained two entries for
"nameserver 127.0.0.53", which defeated our "is resolved actually
in charge" check. Relax that check to allow any number of nameservers,
as long as they're all 127.0.0.53.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/manager_linux.go

@@ -221,8 +221,17 @@ func resolvedIsActuallyResolver(fs wholeFileFS) error {
 	if err != nil {
 		return err
 	}
-	if len(cfg.Nameservers) != 1 || cfg.Nameservers[0] != netaddr.IPv4(127, 0, 0, 53) {
-		return errors.New("resolv.conf doesn't point to systemd-resolved")
+	// We've encountered at least one system where the line
+	// "nameserver 127.0.0.53" appears twice, so we look exhaustively
+	// through all of them and allow any number of repeated mentions
+	// of the systemd-resolved stub IP.
+	if len(cfg.Nameservers) == 0 {
+		return errors.New("resolv.conf has no nameservers")
+	}
+	for _, ns := range cfg.Nameservers {
+		if ns != netaddr.IPv4(127, 0, 0, 53) {
+			return errors.New("resolv.conf doesn't point to systemd-resolved")
+		}
 	}
 	return nil
 }