and current-root-is-target-root (pmax diskimage.
* Add code to check disk names (e.g., "sd0") against the current
kern.root_device sysctl (whic does *not* include a partition).
* Add new function must_mount_root():
check to see if it's the same device as the root. If not,
the current and target root partitions cannot overlap.
If they do, do statfs() on "/", and return 1 iff we get back
"root_device" as the mounted-on filesystem, otherwise zero.
* Call must_mount_root() right after the user specifies a disk as
install/upgrade target, and if it returns 1, print a message asking
them to mount root, and abort.
* Use statfs("/") mounted_from string to check if a disk partition
(e.g., "sd0a") matches the current root.
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