Start no account's manager to look at its units (hq issue 764)

Every 15 s the failed-units look read the own manager of every account
the declaration creates, and reading one is a login that starts it: six
module accounts' managers on the control node started and stopped all
day, most of its journal. A look only reads (ADR 0240 rule 6) and never
asks an account's manager (ADR 0177).

An account's manager is read now only when the declaration places a
user-scoped unit in it, and only after the machine's own manager says
user@<uid>.service is active, as the apply asks before it. A manager not
running is said away in the units statement, with no failed units; in
liveness its units read unknown with the apply's words, never down. The
operator's account on a desktop is still read while logged in, since a
module places a user unit there (issue 315's Razer daemon stays said).

internal/units/exec.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"fmt"
 	"strings"
+
+	"github.com/novox/mesh-host/internal/system"
 )
 
 // Runner runs a command and answers what it printed — the apply's own (apply.ExecRunner), so a look reads
@@ -12,8 +14,9 @@ import (
 type Runner func(ctx context.Context, name string, args ...string) (string, error)
 
 // Exec reads the service managers through systemctl and the package database through the system's own
-// owner query. **Reads only**: `list-units`, `show` and the owner query are the whole of what it asks
-// (ADR 0240 rule 6, and a test holds it).
+// owner query. **Reads only**: `list-units`, `show`, the owner query, and — of the machine's manager and
+// user database — whether an account's manager is active, are the whole of what it asks (ADR 0240 rule 6,
+// and a test holds it).
 type Exec struct {
 	Run Runner
 	// System is what the host was built for; it says how a file's package is asked. A system with no
@@ -103,3 +106,10 @@ func (e Exec) PackageOwning(ctx context.Context, path string) (string, bool, err
 	}
 	return "", false, nil
 }
+
+// UserManagerRunning asks the machine's manager whether an account's own runs, as the apply does before it
+// touches a user-scoped unit (system.UserManagerRunning; novox/hq ADR 0177, issue 764). A system with no
+// manager per account cannot say, and is an error: that manager is said unread.
+func (e Exec) UserManagerRunning(ctx context.Context, account string) (running, exists bool, err error) {
+	return system.ManagerRunning(ctx, e.System, system.Runner(e.Run), account)
+}