tailfs: replace webdavfs with reverse proxies

Instead of modeling remote WebDAV servers as actual
webdav.FS instances, we now just proxy traffic to them.
This not only simplifies the code, but it also allows
WebDAV locking to work correctly by making sure locks are
handled by the servers that need to (i.e. the ones actually
serving the files).

Updates tailscale/corp#16827

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

tailfs/tailfsimpl/dirfs/mkdir.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package dirfs
+
+import (
+	"context"
+	"os"
+
+	"tailscale.com/tailfs/tailfsimpl/shared"
+)
+
+// Mkdir implements webdav.FileSystem. All attempts to Mkdir a directory that
+// already exists will succeed. All other attempts will fail with
+// os.ErrPermission.
+func (dfs *FS) Mkdir(ctx context.Context, name string, perm os.FileMode) error {
+	nameWithoutStaticRoot, isStaticRoot := dfs.trimStaticRoot(name)
+	if isStaticRoot || shared.IsRoot(name) {
+		// root directory already exists, consider this okay
+		return nil
+	}
+
+	child := dfs.childFor(nameWithoutStaticRoot)
+	if child != nil {
+		// child already exists, consider this okay
+		return nil
+	}
+
+	return &os.PathError{Op: "mkdir", Path: name, Err: os.ErrPermission}
+}