feature/taildrop: start moving Taildrop out of LocalBackend

This adds a feature/taildrop package, a ts_omit_taildrop build tag,
and starts moving code to feature/taildrop. In some cases, code
remains where it was but is now behind a build tag. Future changes
will move code to an extension and out of LocalBackend, etc.

Updates #12614

Change-Id: Idf96c61144d1a5f707039ceb2ff59c99f5c1642f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/taildrop/ext.go

@@ -0,0 +1,54 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package taildrop
+
+import (
+	"tailscale.com/ipn/ipnext"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/taildrop"
+	"tailscale.com/tsd"
+	"tailscale.com/types/logger"
+)
+
+func init() {
+	ipnext.RegisterExtension("taildrop", newExtension)
+}
+
+func newExtension(logf logger.Logf, _ *tsd.System) (ipnext.Extension, error) {
+	return &extension{
+		logf: logger.WithPrefix(logf, "taildrop: "),
+	}, nil
+}
+
+type extension struct {
+	logf logger.Logf
+	lb   *ipnlocal.LocalBackend
+	mgr  *taildrop.Manager
+}
+
+func (e *extension) Name() string {
+	return "taildrop"
+}
+
+func (e *extension) Init(h ipnext.Host) error {
+	type I interface {
+		Backend() ipnlocal.Backend
+	}
+	e.lb = h.(I).Backend().(*ipnlocal.LocalBackend)
+
+	// TODO(bradfitz): move init of taildrop.Manager from ipnlocal/peerapi.go to
+	// here
+	e.mgr = nil
+
+	return nil
+}
+
+func (e *extension) Shutdown() error {
+	if mgr, err := e.lb.TaildropManager(); err == nil {
+		mgr.Shutdown()
+	} else {
+		e.logf("taildrop: failed to shutdown taildrop manager: %v", err)
+	}
+	return nil
+}