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_darwin.go

@@ -13,6 +13,6 @@ func newUserspaceRouter(logf logger.Logf, tundev tun.Device, netMon *netmon.Moni
 	return newUserspaceBSDRouter(logf, tundev, netMon)
 }
 
-func cleanup(logger.Logf, string) {
+func cleanUp(logger.Logf, string) {
 	// Nothing to do.
 }