cmd/tailscaled: move cleanup to an implicit action during startup

This removes a potentially increased boot delay for certain boot
topologies where they block on ExecStartPre that may have socket
activation dependencies on other system services (such as
systemd-resolved and NetworkManager).

Also rename cleanup to clean up in affected/immediately nearby places
per code review commentary.

Fixes #11599

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/router/router_windows.go

@@ -120,7 +120,7 @@ func (r *winRouter) Close() error {
 	return nil
 }
 
-func cleanup(logf logger.Logf, interfaceName string) {
+func cleanUp(logf logger.Logf, interfaceName string) {
 	// Nothing to do here.
 }