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

@@ -1606,9 +1606,14 @@ 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 || !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))
@@ -1618,10 +1623,11 @@ int plugin_init(int *argc, char **argv, int flags)
       set the global default storage engine variable so that it will
       not be null in any child thread.
     */
-  global_system_variables.table_plugin=
+    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 */