cmd/tailscale: add background mode to serve/funnel wip (#9202)

> **Note**
> Behind the `TAILSCALE_FUNNEL_DEV` flag

* Expose additional listeners through flags
* Add a --bg flag to run in the background
* --set-path to set a path for a specific target (assumes running in background)

See the parent issue for more context.

Updates #8489

Signed-off-by: Tyler Smalley <tyler@tailscale.com>

cmd/tailscale/cli/funnel.go

@@ -27,7 +27,7 @@ var funnelCmd = func() *ffcli.Command {
 	// implementation of the tailscale funnel command.
 	// See https://github.com/tailscale/tailscale/issues/7844
 	if envknob.UseWIPCode() {
-		return newServeDevCommand(se, "funnel")
+		return newServeDevCommand(se, funnel)
 	}
 	return newFunnelCommand(se)
 }