On server, channel dprint through the readline-device. Also some refactors and quality improvements.

luprex/cpp/drv/readline.hpp

@@ -4,6 +4,7 @@
 
 #include <string>
 #include <string_view>
+#include <mutex>
 #include "drvutil.hpp"
 
 
@@ -12,7 +13,7 @@ public:
     using print_callback = void (*)(const std::u32string &text);
 
 private:
-    print_callback print_cb_;
+    std::mutex mutex_;
     std::u32string desired_command_;
     std::u32string current_command_;
     std::u32string desired_prompt_;
@@ -24,9 +25,6 @@ private:
 
 
 public:
-    // The callback must be set before using the readline device.
-    void set_print_callback(print_callback cb);
-    
     // change the prompt.
     void set_prompt(std::string_view prompt);