Raise a failed unit, the module's and the machine's (hq issue 315)

A module's failed unit now arrives among its resources and raises the
module's own condition, named by the unit. The machine's own failed
units, which no module places, are one warning for the machine listing
them, cleared when none is listed. Nothing a send moved is among them,
so the gate never holds a send on that finding. The statement's units
are kept with the machine's health (migration 0080) and node show says
them.

internal/link/protocol.go

@@ -431,6 +431,42 @@ type Health struct {
 	// Network is the machine's own networking, judged by its engine (novox/hq ADR 0241); nil from an engine
 	// older than that judging, which is "not known", never healthy.
 	Network *NetworkHealth `json:"network,omitempty"`
+	// Units is the machine's service managers as its engine read them (novox/hq issue 315): whether any
+	// unit failed, and each failed unit no module places. A module's failed unit is among Resources, of
+	// kind KindUnit. Nil from an engine older than that reading: not known, never healthy.
+	Units *UnitsHealth `json:"units,omitempty"`
+}
+
+// KindUnit is a module's unit its service manager says failed (issue 315): its package's unit, a unit
+// file it writes, a service whose lifecycle is the machine's — said unhealthy while it stays failed.
+const KindUnit = "unit"
+
+// The states of a machine's service managers (issue 315).
+const (
+	UnitsRunning  = "running"
+	UnitsDegraded = "degraded"
+)
+
+// UnitsHealth is the machine's service managers in one statement (issue 315). The node-engine's own
+// (mesh-host internal/link UnitsHealth).
+type UnitsHealth struct {
+	// State is running, degraded or unknown.
+	State string `json:"state"`
+	// Failed is every unit failed on two looks in a row that no module places: the machine's own.
+	Failed []FailedUnit `json:"failed"`
+	Unread []string     `json:"unread,omitempty"`
+}
+
+// FailedUnit is one failed unit no module places.
+type FailedUnit struct {
+	Unit string `json:"unit"`
+	// Scope is system, or user: an account's own manager.
+	Scope  string `json:"scope"`
+	Load   string `json:"load,omitempty"`
+	Result string `json:"result,omitempty"`
+	// Resource is the mesh's own resource naming it, when the mesh placed it in its own right.
+	Resource string    `json:"resource,omitempty"`
+	Since    time.Time `json:"since"`
 }
 
 // NetworkHealth is a machine's networking in one statement (ADR 0241): the worst of its parts, since