idea: give everyone a mount namespace associated with their job

[wihobbs]

This came up today after the fluxion meeting as I was describing my work with ephemeral and persistent ext4 files in lustre that are mounted/unmounted as a user's job starts/finishes.

@garlick noted that it might be possible for flux to give every user a mount namespace with their flux job that would then allow them to mount userspace filesystems (such as fuse) and the root user to mount other filesystems.

At the end of the job, the root user could more safely unmount the filesystems, with less concern for lingering processes keeping the filesystem internally mounted in the kernel. I think.

[grondo]

Possibly related? #3927