WL#1218 "Triggers". Some very preliminary version of patch.

Mostly needed for Monty for him getting notion what needed for triggers 
from new .FRM format. 

Things to be done:
- Right placement of trigger's invocations
- Right handling of errors in triggers (including transaction rollback)
- Support for priviliges
- Right handling of DROP/RENAME table (hope that it will be handled automatically
  with merging of .TRG into .FRM file)
- Saving/restoring some information critical for trigger creation and replication
  with their definitions (e.g. sql_mode, creator, ...)
- Replication

Already has some known bugs so probably not for general review.

sql/sql_trigger.h

@@ -0,0 +1,62 @@
+/*
+  This class holds all information about triggers of table.
+
+  QQ: Will it be merged into TABLE in future ?
+*/
+class Table_triggers_list: public Sql_alloc
+{
+  /* Triggers as SPs grouped by event, action_time */
+  sp_head           *bodies[3][2];
+  /*
+    Copy of TABLE::Field array with field pointers set to old version
+    of record, used for OLD values in trigger on UPDATE.
+  */
+  Field             **old_field;
+  /*
+    Names of triggers.
+    Should correspond to order of triggers on definitions_list,
+    used in CREATE/DROP TRIGGER for looking up trigger by name.
+  */
+  List<LEX_STRING>  names_list;
+
+public:
+  /*
+    Field responsible for storing triggers definitions in file.
+    It have to be public because we are using it directly from parser.
+  */
+  List<LEX_STRING>  definitions_list;
+
+  Table_triggers_list():
+    old_field(0)
+  {
+    bzero((char *)bodies, sizeof(bodies));
+  }
+  ~Table_triggers_list();
+
+  bool create_trigger(THD *thd, TABLE_LIST *table);
+  bool drop_trigger(THD *thd, TABLE_LIST *table);
+  bool process_triggers(THD *thd, trg_event_type event,
+                        trg_action_time_type time_type)
+  {
+    int res= 0;
+
+    if (bodies[event][time_type])
+    {
+      /*
+        Similar to function invocation we don't need to surpress sending of
+        ok packets here because don't allow execute statements from trigger.
+
+        FIXME: We should juggle with security context here (because trigger
+        should be invoked with creator rights).
+      */
+      res= bodies[event][time_type]->execute_function(thd, 0, 0, 0);
+    }
+
+    return res;
+  }
+
+  static bool check_n_load(THD *thd, const char *db, const char *table_name,
+                           TABLE *table);
+
+  friend class Item_trigger_field;
+};