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:
  sql_protocol_typelib is not exported from libmysqld
  (does not make sense either)
  thus excluded from embedded client
dbug/dbug.c:
  revert changes for Bug#38293
include/my_dbug.h:
  revert changes for Bug#38293
libmysql/libmysql.c:
  Removed DBUG_POP call, because when called after my_end(), will access
  THR_key_mysys that is already deleted. The result of pthread_get_specific
  is not predictable in this case and hence DBUG_POP can crash.
libmysqld/examples/CMakeLists.txt:
  Revert changes for Bug#38293.
libmysqld/lib_sql.cc:
  code to start handle manager is factored out into 
  start_handle_manager() function
libmysqld/libmysqld.def:
  Revert changes for Bug #38293
  Remove excessive exports from libmysqld, export what API documents.
mysys/my_thr_init.c:
  Remove windows-DLL-specific workaround for something (old code, no documentation for
  what specifically). The problem is that even after my_thread_end() is finished, 
  DBUG statement can initiate my_thread_init(). This does not happen anywhere else and 
  should not happen on  Windows either.
sql/mysql_priv.h:
  - new functions start_handle_manager() and stop_handle_manager()
  - move manager_thread_in_use  variable to sql_manager.cc and made
  it static
  - remove manager_status, as it is unused
sql/mysqld.cc:
  Code to start/stop handle_manager thread is factored out into start_handle_manager()

libmysqld/lib_sql.cc

@@ -386,6 +386,7 @@ static void emb_free_embedded_thd(MYSQL *mysql)
   thd->store_globals();
   thd->unlink();
   delete thd;
+  my_pthread_setspecific_ptr(THR_THD,  0);
   mysql->thd=0;
 }
 
@@ -539,12 +540,7 @@ int init_embedded_server(int argc, char **argv, char **groups)
 
   (void) thr_setconcurrency(concurrency);	// 10 by default
 
-  if (flush_time && flush_time != ~(ulong) 0L)
-  {
-    pthread_t hThread;
-    if (pthread_create(&hThread,&connection_attrib,handle_manager,0))
-      sql_print_error("Warning: Can't create thread to manage maintenance");
-  }
+  start_handle_manager();
 
   // FIXME initialize binlog_filter and rpl_filter if not already done
   //       corresponding delete is in clean_up()