Backport from 6.0-codebase.

WL#3771
  "Audit Plugin Interface"
  Implement new plug-in type - AUDIT
  New plug-in: audit_null
    simply increments counter for how many times it was called.

sql/sql_class.h

@@ -23,6 +23,7 @@
 #pragma interface			/* gcc class implementation */
 #endif
 
+#include <mysql/plugin_audit.h>
 #include "log.h"
 #include "rpl_tblmap.h"
 
@@ -1803,6 +1804,20 @@ public:
   partition_info *work_part_info;
 #endif
 
+#ifndef EMBEDDED_LIBRARY
+  /**
+    Array of active audit plugins which have been used by this THD.
+    This list is later iterated to invoke release_thd() on those
+    plugins.
+  */
+  DYNAMIC_ARRAY audit_class_plugins;
+  /**
+    Array of bits indicating which audit classes have already been
+    added to the list of audit plugins which are currently in use.
+  */
+  unsigned long audit_class_mask[MYSQL_AUDIT_CLASS_MASK_SIZE];
+#endif
+
 #if defined(ENABLED_DEBUG_SYNC)
   /* Debug Sync facility. See debug_sync.cc. */
   struct st_debug_sync_control *debug_sync_control;