cmd/cloner: deep-clone pointer elements in map-of-slice values

The cloner's codegen for map[K][]*V fields was doing a shallow
append (copying pointer values) instead of cloning each element.
This meant that cloned structs aliased the original's pointed-to
values through the map's slice entries.

Mirror the existing standalone-slice logic that checks
ContainsPointers(sliceType.Elem()) and generates per-element
cloning for pointer, interface, and struct types.

Regenerate net/dns and tailcfg which both had affected
map[...][]*dnstype.Resolver fields.

Fixes #19284

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

tailcfg/tailcfg_clone.go

@@ -262,8 +262,18 @@ func (src *DNSConfig) Clone() *DNSConfig {
 	}
 	if dst.Routes != nil {
 		dst.Routes = map[string][]*dnstype.Resolver{}
-		for k := range src.Routes {
-			dst.Routes[k] = append([]*dnstype.Resolver{}, src.Routes[k]...)
+		for k, sv := range src.Routes {
+			if sv == nil {
+				continue
+			}
+			dst.Routes[k] = make([]*dnstype.Resolver, len(sv))
+			for i := range sv {
+				if sv[i] == nil {
+					dst.Routes[k][i] = nil
+				} else {
+					dst.Routes[k][i] = sv[i].Clone()
+				}
+			}
 		}
 	}
 	if src.FallbackResolvers != nil {