MDEV-9566 Server code changes in preparation for mariabackup

- Backup will  load encryption plugins outside of mysqld. Thus, do not
force loading MyISAM plugin in plugin_load.

- init_signals() will be used in backup,  make it global, not static.

sql/mysqld.cc

@@ -3100,7 +3100,7 @@ LONG WINAPI my_unhandler_exception_filter(EXCEPTION_POINTERS *ex_pointers)
 }
 
 
-static void init_signals(void)
+void init_signals(void)
 {
   if(opt_console)
     SetConsoleCtrlHandler(console_event_handler,TRUE);
@@ -3231,7 +3231,7 @@ static size_t my_setstacksize(pthread_attr_t *attr, size_t stacksize)
 
 #ifndef EMBEDDED_LIBRARY
 
-static void init_signals(void)
+void init_signals(void)
 {
   sigset_t set;
   struct sigaction sa;

sql/sql_plugin.cc

@@ -1606,22 +1606,28 @@ int plugin_init(int *argc, char **argv, int flags)
     }
   }
 
-  /* First, we initialize only MyISAM - that should always succeed */
+  /*
+    First, we initialize only MyISAM - that should almost always succeed
+    (almost always, because plugins can be loaded outside of the server, too).
+  */
   plugin_ptr= plugin_find_internal(&MyISAM, MYSQL_STORAGE_ENGINE_PLUGIN);
-  DBUG_ASSERT(plugin_ptr);
-  DBUG_ASSERT(plugin_ptr->load_option == PLUGIN_FORCE);
+  DBUG_ASSERT(plugin_ptr || !mysql_mandatory_plugins[0]);
+  if (plugin_ptr)
+  {
+    DBUG_ASSERT(plugin_ptr->load_option == PLUGIN_FORCE);
 
-  if (plugin_initialize(&tmp_root, plugin_ptr, argc, argv, false))
-    goto err_unlock;
+    if (plugin_initialize(&tmp_root, plugin_ptr, argc, argv, false))
+      goto err_unlock;
 
-  /*
-    set the global default storage engine variable so that it will
-    not be null in any child thread.
-  */
-  global_system_variables.table_plugin=
-    intern_plugin_lock(NULL, plugin_int_to_ref(plugin_ptr));
-  DBUG_ASSERT(plugin_ptr->ref_count == 1);
+    /*
+      set the global default storage engine variable so that it will
+      not be null in any child thread.
+    */
+    global_system_variables.table_plugin =
+      intern_plugin_lock(NULL, plugin_int_to_ref(plugin_ptr));
+      DBUG_ASSERT(plugin_ptr->ref_count == 1);
 
+  }
   mysql_mutex_unlock(&LOCK_plugin);
 
   /* Register (not initialize!) all dynamic plugins */