This allows avoiding having to delete after the fact for things like
issues #57 and #98.
Why have this over just stopping clipmenud? Well:
1. Stopping clipmenud should usually be an init system action, but we
are init-system agnostic. If we just exit, we don't have a way of
reliably starting again.
2. Even if we *do* do it using the init system, we don't want some
things (like a lingering xsel which owns the selection for
CM_OWN_CLIPBOARD) being killed as well.
3. This is a nicer interface for things like password managers to stop
clipmenu rather than stopping clipmenu entirely.
