client/Process: explicitly disallow "idle" and "noidle" in command lists

These commands cannot possibly work with command lists because command
lists are supposed to be atomic, but suspended command execution
conflicts with that.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/2167

src/client/Process.cxx

@@ -52,6 +52,13 @@ Client::ProcessCommandList(bool list_ok,
 	return CommandResult::OK;
 }
 
+[[gnu::pure]]
+static bool
+IsAsyncCommmand(const char *line) noexcept
+{
+	return StringIsEqual(line, "idle") || StringIsEqual(line, "noidle");
+}
+
 CommandResult
 Client::ProcessLine(char *line) noexcept
 {
@@ -67,6 +74,13 @@ Client::ProcessLine(char *line) noexcept
 		return CommandResult::CLOSE;
 	}
 
+	if (cmd_list.IsActive() && IsAsyncCommmand(line)) {
+		FmtWarning(client_domain,
+			   "[{}] not possible in comand list: \"{}\"",
+			   num, line);
+		return CommandResult::CLOSE;
+	}
+
 	if (StringIsEqual(line, "noidle")) {
 		if (idle_waiting) {
 			/* send empty idle response and leave idle mode */