net/dns/resolver: add live reconfig, plumb through to ipnlocal.

The resolver still only supports a single upstream config, and
ipn/wgengine still have to split up the DNS config, but this moves
closer to unifying the DNS configs.

As a handy side-effect of the refactor, IPv6 MagicDNS records exist
now.

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

wgengine/wgengine.go

@@ -9,7 +9,6 @@ import (
 
 	"inet.af/netaddr"
 	"tailscale.com/ipn/ipnstate"
-	"tailscale.com/net/dns/resolver"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine/filter"
@@ -57,7 +56,7 @@ type Engine interface {
 	// sends an updated network map.
 	//
 	// The returned error is ErrNoChanges if no changes were made.
-	Reconfig(*wgcfg.Config, *router.Config) error
+	Reconfig(*wgcfg.Config, *router.Config, map[string][]netaddr.IP, []string) error
 
 	// GetFilter returns the current packet filter, if any.
 	GetFilter() *filter.Filter
@@ -65,9 +64,6 @@ type Engine interface {
 	// SetFilter updates the packet filter.
 	SetFilter(*filter.Filter)
 
-	// SetDNSMap updates the DNS map.
-	SetDNSMap(*resolver.Map)
-
 	// SetStatusCallback sets the function to call when the
 	// WireGuard status changes.
 	SetStatusCallback(StatusCallback)