zfs: declare health on the four running services, README says hand acts answer started

Builds: design 42 zfs-owns-zfs §13, §24, lead answers A-45, A-43

modules/zfs/README.md

@@ -92,7 +92,7 @@ runs out of time is ended with everything it started.
 | `zfs_config` | every value with its config value, what runs, how it stands, the package's files, and what the module says before it leaves |
 | `zfs_config_apply` | converge now, as the apply does |
 | `zfs_snapshots_schedule` | the keep counts, next moments, what the next run would remove, and the ledger |
-| `zfs_snapshots_run_now`, `zfs_snapshots_prune_now`, `zfs_snapshots_pause`, `zfs_snapshots_resume` | a person's acts on the scheduled ZFS snapshots; the hand acts wait up to 30 seconds for their event, else answer "started" |
+| `zfs_snapshots_run_now`, `zfs_snapshots_prune_now`, `zfs_snapshots_pause`, `zfs_snapshots_resume` | a person's acts on the scheduled ZFS snapshots; the hand acts answer "started" at once and `zfs_module_events` reads the outcome |
 | `zfs_module_events` | the module's own events, newest first (`zfs_events` reads ZFS's) |
 | `zfs_snapshots_check` | the ledger against what ZFS holds; changes nothing |
 | `zfs_findings` | the follower's findings |

modules/zfs/module.json

@@ -479,21 +479,30 @@
       "type": "service",
       "unit": "zfs-import-cache.service",
       "state": "running",
-      "boot": "enabled"
+      "boot": "enabled",
+      "health": {
+        "kind": "unit"
+      }
     },
     {
       "id": "mount",
       "type": "service",
       "unit": "zfs-mount.service",
       "state": "running",
-      "boot": "enabled"
+      "boot": "enabled",
+      "health": {
+        "kind": "unit"
+      }
     },
     {
       "id": "target",
       "type": "service",
       "unit": "zfs.target",
       "state": "running",
-      "boot": "enabled"
+      "boot": "enabled",
+      "health": {
+        "kind": "unit"
+      }
     },
     {
       "id": "import-scan",
@@ -530,7 +539,10 @@
         "zed-handler-all-syslog",
         "zed-handler-scrub-after-resilver",
         "zed-drop-in"
-      ]
+      ],
+      "health": {
+        "kind": "unit"
+      }
     },
     {
       "id": "zed-off",