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/keeper_linux.go

@@ -272,8 +272,8 @@ func TakeFolder(ctx context.Context, d *declaration.Declaration, id, path, warra
 			f.Kept, f.How, f.At, f.Users = true, store.KeptByWarrant, started, nil
 		}
 		f.Warrant, f.By, f.Mount, f.Want, f.IDs = warrant, by, v.mount, users, want.ids
-		// The warrant is for the filesystem it was carried out on (§8).
-		f.TakenOn = v.mount
+		// The warrant is for the filesystem it was carried out on (§8), named as a wait names it.
+		f.Placed, f.TakenOn = PlacedID(v.mount), PlacedID(v.mount)
 		f.Act = &store.KeptAct{Act: "take", Warrant: warrant, By: by, State: "running", Started: started}
 		return true
 	})