feature/drive: start factoring out Taildrive, add ts_omit_drive build tag

As of this commit (per the issue), the Taildrive code remains where it
was, but in new files that are protected by the new ts_omit_drive
build tag. Future commits will move it.

Updates #17058

Change-Id: Idf0a51db59e41ae8da6ea2b11d238aefc48b219e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/drive_tomove.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// This is the Taildrive stuff that should ideally be registered in init only when
+// the ts_omit_drive is not set, but for transition reasons is currently (2025-09-08)
+// always defined, as we work to pull it out of LocalBackend.
+
+package ipnlocal
+
+import "tailscale.com/tailcfg"
+
+const (
+	// DriveLocalPort is the port on which the Taildrive listens for location
+	// connections on quad 100.
+	DriveLocalPort = 8080
+)
+
+// DriveSharingEnabled reports whether sharing to remote nodes via Taildrive is
+// enabled. This is currently based on checking for the drive:share node
+// attribute.
+func (b *LocalBackend) DriveSharingEnabled() bool {
+	return b.currentNode().SelfHasCap(tailcfg.NodeAttrsTaildriveShare)
+}
+
+// DriveAccessEnabled reports whether accessing Taildrive shares on remote nodes
+// is enabled. This is currently based on checking for the drive:access node
+// attribute.
+func (b *LocalBackend) DriveAccessEnabled() bool {
+	return b.currentNode().SelfHasCap(tailcfg.NodeAttrsTaildriveAccess)
+}