Leave no leftover for a folder kept only while empty, and name filesystems by more than their mount point (hq ADR 0325 third review)

A folder kept because it was empty, then covered by its disk, was said as unhealthy
leftover entries for ever: its entries sit on a hidden empty folder and grant nothing,
so they are now forgotten, and only entries walked onto data under a warrant are kept
to be taken out. A warrant and a kept folder now name their filesystem by mount point,
source and type, so another pool at the same point is a move; records naming a mount
point are read as its name. A record with no access ids is said under a path-free name.

internal/apply/placed.go

@@ -147,7 +147,11 @@ func mountinfoPoints(raw string) []string {
 
 // mountSourceOf is the source and filesystem type of the mount at point, from mountinfo: for a ZFS dataset, its
 // name. Empty when the machine says nothing of it.
-func mountSourceOf(point string) (source, fstype string) {
+func mountSourceOf(point string) (source, fstype string) { return MountSource(point) }
+
+// MountSource is mountSourceOf as the machine answers it. A variable so a test can stand in for another filesystem
+// mounted at the same point.
+var MountSource = func(point string) (source, fstype string) {
 	raw, err := os.ReadFile("/proc/self/mountinfo")
 	if err != nil {
 		return "", ""