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.go

@@ -79,16 +79,20 @@ func Shutdown() error {
 	return err
 }
 
-// LogToTerminal sends all logging to stdout as coloured text, replacing the
-// default slog logger. When stdout is an interactive terminal, loggers
-// derived from it report so to WritesToTerminal.
+// LogToTerminal sends all logging to stdout as text, replacing the default
+// slog logger. The text is coloured only on an interactive terminal, so a
+// pipe, a file or the systemd journal gets no escape codes. On an
+// interactive terminal, loggers derived from it report so to
+// WritesToTerminal.
 func LogToTerminal() {
+	interactive := isTerminal(terminal)
 	opts := &tint.Options{
 		Level:      &logLevel,
 		TimeFormat: "*",
+		NoColor:    !interactive,
 	}
 	handler := tint.NewTextHandler(terminal, opts)
-	if isTerminal(terminal) {
+	if interactive {
 		handler = terminalHandler{
 			Handler: handler,
 		}