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_unix.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build unix
+
+package tailfs
+
+import "tailscale.com/version"
+
+func doAllowShareAs() bool {
+	// All UNIX platforms use user servers (sub-processes) to access the OS
+	// filesystem as a specific unprivileged users, except for sandboxed macOS
+	// which doesn't support impersonating users and instead accesses files
+	// through the macOS GUI app as whatever unprivileged user is running it.
+	return !version.IsSandboxedMacOS()
+}