Serve reset-failed and wanted-by in the systemd module (hq issue 332)

So a leftover unit's trigger is found and its failed record cleared
through the seat, not a shell: wanted-by reads the manager's reverse
dependencies and the enable links naming the unit, saying which dangle.

modules/systemd/cmd/systemd-tools/client.go

@@ -98,7 +98,7 @@ func execRunner(cmd string, args []string, env []string) Ran {
 const MeshUnitHeader = "# Generated by the mesh."
 
 // acts are the verbs that change the system manager's state, which polkit keeps from a non-root account.
-var acts = map[string]bool{"start": true, "stop": true, "restart": true, "enable": true, "disable": true}
+var acts = map[string]bool{"start": true, "stop": true, "restart": true, "enable": true, "disable": true, "reset-failed": true}
 
 // escalated is the command as it is run: as given when this process is root, or in the user scope, or
 // when the call is a systemctl read; else through sudo without a prompt — an act on the system manager,
@@ -137,6 +137,10 @@ type Manager struct {
 	Read func(path string) (string, error)
 	// Env is this process's environment, the base of a user-scope call's.
 	Env []string
+	// Home is the operator account's home, whose configuration holds its own enable links.
+	Home string
+	// ReadDir lists a directory, to find the enable links naming a unit (wanted.go).
+	ReadDir func(dir string) ([]Entry, error)
 }
 
 var userBus = regexp.MustCompile(`(?i)Failed to connect to (user scope )?bus`)
@@ -267,7 +271,8 @@ func (m *Manager) writtenByMesh(path string) bool {
 	return err == nil && strings.HasPrefix(text, MeshUnitHeader)
 }
 
-// Act starts, stops, restarts, enables or disables one unit, and answers with the state after.
+// Act starts, stops, restarts, enables or disables one unit, or resets its failed record, and answers with
+// the state after.
 func (m *Manager) Act(scope Scope, verb, unit string) (map[string]any, error) {
 	if err := unitArg(unit); err != nil {
 		return nil, err