Name a filesystem by its kernel id, never its device name (hq ADR 0325 fourth review)

An ext4 or xfs library mounted by UUID can come back from a reboot as another device
(/dev/sdb1 as /dev/sdc1); naming the filesystem by mountinfo's source read that as a
move, stripped the warranted library's entries and asked for a new Approve. A
filesystem is now named by statfs's f_fsid (from the UUID, or the ZFS dataset), its
type and its mount point. Entries left on the filesystem a folder is on now are
merged back or held, never walked away, and a TakenOn naming a mount point is not
taken.

internal/apply/walk_other.go

@@ -53,3 +53,6 @@ type Watcher struct{}
 func NewWatcher() (*Watcher, error)              { return nil, errNoKeeping }
 func (*Watcher) Set(map[string][]store.KeptUser) {}
 func (*Watcher) Run(context.Context)             {}
+
+// MountFsid reads no filesystem id where a folder is not kept.
+var MountFsid = func(string) string { return "" }