(#4914) Prefetch mountstate

Add a method mountinstances that returns a list of currently mounted
filesystems and overwrite prefetch to update the :ensure state of a
mount resource to either

• :absent => not in fstab and not mounted
• :unmounted => in fstab but not mounted
• :mounted => in fstab and mounted
• :ghost => not in fstab but mounted

This is just one step towards 4914. Next will be query the property_hash
when asking mounted? and update the insync? methods to handle the
different states.