GOption can handle --help for us, so we don't need to reimplement this
ourselves. This causes a small regression as starting a main
gnome-control-center instance and then running gnome-control-center
--help will cause the main instance-control-center to exit. This will be
fixed in the following patches, and this fixes the opposite bug:
if gnome-control-center is not running, gnome-control-center --help
would not exit after displaying the help before this commit.

https://bugzilla.gnome.org/show_bug.cgi?id=751597