tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/compositefs/removeall.go

@@ -0,0 +1,33 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package compositefs
+
+import (
+	"context"
+	"os"
+
+	"tailscale.com/tailfs/shared"
+)
+
+// RemoveAll implements webdav.File. The root of this file system is read-only,
+// so attempting to call RemoveAll on the root will fail with os.ErrPermission.
+// RemoveAll within a child will be handled by the respective child.
+func (cfs *CompositeFileSystem) RemoveAll(ctx context.Context, name string) error {
+	if shared.IsRoot(name) {
+		// root directory is read-only
+		return os.ErrPermission
+	}
+
+	pathInfo, err := cfs.pathInfoFor(name)
+	if pathInfo.refersToChild {
+		// children can't be removed
+		return os.ErrPermission
+	}
+
+	if err != nil {
+		return err
+	}
+
+	return pathInfo.child.FS.RemoveAll(ctx, pathInfo.pathOnChild)
+}