RFC: possible issue with kqueue

[Rozhuk Ivan]

> I wonder if a reasonable fix might be to have some sort of pre-unmount
> event that can be delivered via kqueue, so that a userland entity
> monitoring on that volume has a chance to close related descriptors so
> that the unmount can proceed?
> 

https://reviews.freebsd.org/D19690

I do not finish this.
Plan was: implement this and catch unmount notify in my FAM, that in glib20.