osx: installer will fail if the bitmask_helper is loaded but not running

currently we only try to unload the bitmask_helper if it is running. there is a corner case, when the launchdaemon is loaded but is not running. this can happen, for instance, if there is an error with the paths, permissions, etc. a sympton of this is that riseupvpn won't be able to launch (is bitmaskd running?). either the preinstall hook needs to try harder, or we try to unload the daemon on the uninstall script.