wgengine/router: restore /etc/resolv.conf after tailscale down is called

This change is to restore /etc/resolv.conf after tailscale down is called. This is done by setting the dns.Manager before errors occur. Error collection is also added.

Fixes #723

wgengine/router/router_linux.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/coreos/go-iptables/iptables"
+	"github.com/go-multierror/multierror"
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun"
 	"inet.af/netaddr"
@@ -145,7 +146,7 @@ func (r *linuxRouter) Up() error {
 
 func (r *linuxRouter) Close() error {
 	if err := r.dns.Down(); err != nil {
-		return fmt.Errorf("dns down: %v", err)
+		return fmt.Errorf("dns down: %w", err)
 	}
 	if err := r.downInterface(); err != nil {
 		return err
@@ -165,23 +166,28 @@ func (r *linuxRouter) Close() error {
 
 // Set implements the Router interface.
 func (r *linuxRouter) Set(cfg *Config) error {
+	var errs []error
 	if cfg == nil {
 		cfg = &shutdownConfig
 	}
 
+	if err := r.dns.Set(cfg.DNS); err != nil {
+		errs = append(errs, fmt.Errorf("dns set: %w", err))
+	}
+
 	if err := r.setNetfilterMode(cfg.NetfilterMode); err != nil {
-		return err
+		errs = append(errs, err)
 	}
 
 	newRoutes, err := cidrDiff("route", r.routes, cfg.Routes, r.addRoute, r.delRoute, r.logf)
 	if err != nil {
-		return err
+		errs = append(errs, err)
 	}
 	r.routes = newRoutes
 
 	newAddrs, err := cidrDiff("addr", r.addrs, cfg.LocalAddrs, r.addAddress, r.delAddress, r.logf)
 	if err != nil {
-		return err
+		errs = append(errs, err)
 	}
 	r.addrs = newAddrs
 
@@ -190,20 +196,16 @@ func (r *linuxRouter) Set(cfg *Config) error {
 		// state already correct, nothing to do.
 	case cfg.SNATSubnetRoutes:
 		if err := r.addSNATRule(); err != nil {
-			return err
+			errs = append(errs, err)
 		}
 	default:
 		if err := r.delSNATRule(); err != nil {
-			return err
+			errs = append(errs, err)
 		}
 	}
 	r.snatSubnetRoutes = cfg.SNATSubnetRoutes
 
-	if err := r.dns.Set(cfg.DNS); err != nil {
-		return fmt.Errorf("dns set: %v", err)
-	}
-
-	return nil
+	return multierror.New(errs)
 }
 
 // setNetfilterMode switches the router to the given netfilter