Make disks show info about unmounted disks as well, first effort

[CongLuanTran]

We clone the keys of procfs_disk_stats to create a list of all disks, including unmounted ones, and let's call that unmounted. Then after adding the mounted disks (and at the same time removing those disks from the unmounted), we can iterate over unmounted and add the remaining disks.

Unmounted disks will have "Unknown" file system and empty mount point, except Zram, which has "swap" file system and "[SWAP]" mount point (like lsblk).

[GuillaumeGomez]

Not sure if it's really useful but if you're willing to implement it for all sysinfo supported systems, I'm fine with it.

[GuillaumeGomez]

With this new feature, I think mount_point should become an enum looking like this:

enum MountPoint {
    Path(Path)
    Swap,
    Unmounted,
}

[CongLuanTran]

I don't have a Mac (and don't know it's system), so I doubt I can implement for it. But I can try with Windows.