fix for bug #17619 Scheduler race conditions

- Scheduler is either initialized at server start or never.
  Starting & stopping is now suspending & resuming.
- The scheduler has clear OO interface
- Now all calls to the scheduler are synchronous
- GLOBAL event_scheduler uses thd::sys_var_tmp (see set_var.cc)
- External API is encapsulated into class Events
- Includes fixes for all comments of Kostja's review of 19.05.2005

Starting to merge into 5.1-release (5.1.10) and push

sql/sql_class.h

@@ -629,6 +629,8 @@ public:
   {
     return (*priv_host ? priv_host : (char *)"%");
   }
+  
+  bool set_user(char *user_arg);
 };
 
 
@@ -770,6 +772,19 @@ public:
 };
 
 
+/* Flags for the THD::system_thread variable */
+enum enum_thread_type
+{
+  NON_SYSTEM_THREAD= 0,
+  SYSTEM_THREAD_DELAYED_INSERT= 1,
+  SYSTEM_THREAD_SLAVE_IO= 2,
+  SYSTEM_THREAD_SLAVE_SQL= 4,
+  SYSTEM_THREAD_NDBCLUSTER_BINLOG= 8,
+  SYSTEM_THREAD_EVENT_SCHEDULER= 16,
+  SYSTEM_THREAD_EVENT_WORKER= 32
+};
+
+
 /*
   For each client connection we create a separate thread with THD serving as
   a thread/connection descriptor
@@ -1103,7 +1118,8 @@ public:
   long	     dbug_thread_id;
   pthread_t  real_id;
   uint	     tmp_table, global_read_lock;
-  uint	     server_status,open_options,system_thread;
+  uint	     server_status,open_options;
+  enum enum_thread_type system_thread;
   uint32     db_length;
   uint       select_number;             //number of select (used for EXPLAIN)
   /* variables.transaction_isolation is reset to this after each commit */
@@ -1404,11 +1420,6 @@ public:
 
 #define reenable_binlog(A)   (A)->options= tmp_disable_binlog__save_options;}
 
-/* Flags for the THD::system_thread (bitmap) variable */
-#define SYSTEM_THREAD_DELAYED_INSERT 1
-#define SYSTEM_THREAD_SLAVE_IO 2
-#define SYSTEM_THREAD_SLAVE_SQL 4
-#define SYSTEM_THREAD_NDBCLUSTER_BINLOG 8
 
 /*
   Used to hold information about file and file structure in exchainge