Bug#38522: 5 seconds delay when closing application using embedded server

The problem here is that embedded server starts handle_thread manager 
thread  on mysql_library_init() does not stop it on mysql_library_end().
At shutdown, my_thread_global_end() waits for thread count to become 0,
but since we did not stop the thread it will give up after 5 seconds.
             
Solution is to move shutdown for handle_manager thread from kill_server()
(mysqld specific) to clean_up() that is used by both embedded and mysqld.
            
This patch also contains some refactorings - to avoid duplicate code,
start_handle_manager() and stop_handle_manager() functions are introduced.
Unused variables are eliminated. handle_manager does not rely on global
variable abort_loop anymore to stop (abort_loop is not set for embedded).
            
Note: Specifically on Windows and when using DBUG version of libmysqld, 
the complete solution requires removing obsolete code my_thread_init() 
from my_thread_var(). This has a side effect that a DBUG statement 
after my_thread_end() can cause thread counter to be incremented, and 
embedded will hang for some seconds. Or worse, my_thread_init() will 
crash if critical sections have been deleted by the global cleanup 
routine that runs in a different thread. 

This patch also fixes and revert prior changes for Bug#38293 
"Libmysqld crash in mysql_library_init if language file missing".

Root cause of the crash observed in Bug#38293  was bug in my_thread_init() 
described above

client/mysql.cc

@@ -1627,8 +1627,10 @@ get_one_option(int optid, const struct my_option *opt __attribute__((unused)),
     opt_nopager= 1;
     break;
   case OPT_MYSQL_PROTOCOL:
+#ifndef EMBEDDED_LIBRARY
     opt_protocol= find_type_or_exit(argument, &sql_protocol_typelib,
                                     opt->name);
+#endif
     break;
   case OPT_SERVER_ARG:
 #ifdef EMBEDDED_LIBRARY