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>

tsnet/tsnet.go

@@ -530,7 +530,8 @@ func (s *Server) start() (reterr error) {
 	closePool.add(s.dialer)
 	sys.Set(eng)
 
-	ns, err := netstack.Create(logf, sys.Tun.Get(), eng, sys.MagicSock.Get(), s.dialer, sys.DNSManager.Get(), sys.ProxyMapper())
+	// TODO(oxtoacart): do we need to support Tailfs on tsnet, and if so, how?
+	ns, err := netstack.Create(logf, sys.Tun.Get(), eng, sys.MagicSock.Get(), s.dialer, sys.DNSManager.Get(), sys.ProxyMapper(), nil)
 	if err != nil {
 		return fmt.Errorf("netstack.Create: %w", err)
 	}