Terminal logging is now coloured only on an interactive terminal

The terminal logger always coloured its text, so output piped or
redirected to a file, and the systemd journal, got escape codes. It
now colours only an interactive terminal.

The service's logs can therefore go to the journal: logging.file is
now optional for it, and the unit's comments and the README say so.

internal/logging/logging_test.go

@@ -171,6 +171,36 @@ func TestLogToTerminal(t *testing.T) {
 	}
 }
 
+// The text is coloured on an interactive terminal only.
+func TestLogToTerminalColour(t *testing.T) {
+	for _, interactive := range []bool{true, false} {
+		saveGlobals(t)
+		out, err := os.CreateTemp(t.TempDir(), "stdout")
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Cleanup(func() {
+			out.Close()
+		})
+		terminal = out
+		isTerminal = func(*os.File) bool {
+			return interactive
+		}
+
+		LogToTerminal()
+		slog.Warn("hello", "key", "value")
+
+		written, err := os.ReadFile(out.Name())
+		if err != nil {
+			t.Fatal(err)
+		}
+		coloured := strings.Contains(string(written), "\x1b[")
+		if coloured != interactive || !strings.Contains(string(written), "hello") {
+			t.Errorf("interactive %v: wrote %q", interactive, written)
+		}
+	}
+}
+
 // On an interactive terminal the handler, and loggers derived from it, report
 // that they write to the terminal; when stdout is a file or pipe they don't.
 func TestWritesToTerminal(t *testing.T) {