wgengine/magicsock: fix js/wasm crash regression loading non-existent portmapper

Thanks for the report, @Need-an-AwP!

Fixes #17681
Updates #9394

Change-Id: I2e0b722ef9b460bd7e79499192d1a315504ca84c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/tailscale/apitype/apitype.go

@@ -94,3 +94,13 @@ type DNSQueryResponse struct {
 	// Resolvers is the list of resolvers that the forwarder deemed able to resolve the query.
 	Resolvers []*dnstype.Resolver
 }
+
+// OptionalFeatures describes which optional features are enabled in the build.
+type OptionalFeatures struct {
+	// Features is the map of optional feature names to whether they are
+	// enabled.
+	//
+	// Disabled features may be absent from the map. (That is, false values
+	// are not guaranteed to be present.)
+	Features map[string]bool
+}