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/remote_nonunix.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !unix
+
+package tailfs
+
+func doAllowShareAs() bool {
+	// On non-UNIX platforms, we use the GUI application (e.g. Windows taskbar
+	// icon) to access the filesystem as whatever unprivileged user is running
+	// the GUI app, so we cannot allow sharing as a different user.
+	return false
+}