Compare mount by value instead of reference

[NymanRobin]

This has to be since the mounts are reloaded each time a mount is added. In case of two mounts mounting at the same time there will be a race condition for applying policy.

I added the unit test in mountpoint for my sanity while developing and some kind of documentation of the issue.

This fixes issue #411

[NymanRobin]

Thanks for the comments, I fixed them now!

[NymanRobin]

Thanks for the comments @ebiggers, I fixed up the stuff. Hope it looks good now :)