mount / umount replacement

[flihp]

The current secret / auth import method (aka how we get key shares and auth values from the operator) relies on the mount and umount commands. Like #262 we'd rather call the appropriate IOCTL directly. In the case of mount the folks over at pop-os have already wrapped this IOCTL up into a crate: https://docs.rs/sys-mount/latest/sys_mount/