This will be pushed only after I fix the testsuite.

This is the main commit for Worklog tasks:
 * A more dynamic binlog format which allows small changes (1064)
 * Log session variables in Query_log_event (1063)
Below 5.0 means 5.0.0.
MySQL 5.0 is able to replicate FOREIGN_KEY_CHECKS, UNIQUE_KEY_CHECKS (for speed),
SQL_AUTO_IS_NULL, SQL_MODE. Not charsets (WL#1062), not some vars (I can only think
of SQL_SELECT_LIMIT, which deserves a special treatment). Note that this
works for queries, except LOAD DATA INFILE (for this it would have to wait
for Dmitri's push of WL#874, which in turns waits for the present push, so...
the deadlock must be broken!). Note that when Dmitri pushes WL#874 in 5.0.1,
5.0.0 won't be able to replicate a LOAD DATA INFILE from 5.0.1.
Apart from that, the new binlog format is designed so that it can tolerate
a little variation in the events (so that a 5.0.0 slave could replicate a
5.0.1 master, except for LOAD DATA INFILE unfortunately); that is, when I
later add replication of charsets it should break nothing. And when I later
add a UID to every event, it should break nothing.
The main change brought by this patch is a new type of event, Format_description_log_event,
which describes some lengthes in other event types. This event is needed for
the master/slave/mysqlbinlog to understand a 5.0 log. Thanks to this event,
we can later add more bytes to the header of every event without breaking compatibility.
Inside Query_log_event, we have some additional dynamic format, as every Query_log_event
can have a different number of status variables, stored as pairs (code, value); that's
how SQL_MODE and session variables and catalog are stored. Like this, we can later
add count of affected rows, charsets... and we can have options --don't-log-count-affected-rows
if we want.
MySQL 5.0 is able to run on 4.x relay logs, 4.x binlogs.
Upgrading a 4.x master to 5.0 is ok (no need to delete binlogs),
upgrading a 4.x slave to 5.0 is ok (no need to delete relay logs);
so both can be "hot" upgrades.
Upgrading a 3.23 master to 5.0 requires as much as upgrading it to 4.0.
3.23 and 4.x can't be slaves of 5.0.
So downgrading from 5.0 to 4.x may be complicated.
Log_event::log_pos is now the position of the end of the event, which is
more useful than the position of the beginning. We take care about compatibility
with <5.0 (in which log_pos is the beginning).
I added a short test for replication of SQL_MODE and some other variables.
TODO:
- after committing this, merge the latest 5.0 into it
- fix all tests
- update the manual with upgrade notes.

sql/sql_class.h

@@ -26,6 +26,7 @@
 class Query_log_event;
 class Load_log_event;
 class Slave_log_event;
+class Format_description_log_event;
 class sp_rcontext;
 class sp_cache;
 
@@ -99,7 +100,14 @@ class MYSQL_LOG
   enum cache_type io_cache_type;
   bool write_error, inited;
   bool need_start_event;
-  bool no_auto_events;				// For relay binlog
+  /*
+    no_auto_events means we don't want any of these automatic events :
+    Start/Rotate/Stop. That is, in 4.x when we rotate a relay log, we don't want
+    a Rotate_log event to be written to the relay log. When we start a relay log
+    etc. So in 4.x this is 1 for relay logs, 0 for binlogs.
+    In 5.0 it's 0 for relay logs too!
+  */
+  bool no_auto_events;			     
   /* 
      The max size before rotation (usable only if log_type == LOG_BIN: binary
      logs and relay logs).
@@ -116,6 +124,18 @@ class MYSQL_LOG
 public:
   MYSQL_LOG();
   ~MYSQL_LOG();
+
+  /* 
+     These describe the log's format. This is used only for relay logs.
+     _for_exec is used by the SQL thread, _for_queue by the I/O thread. It's
+     necessary to have 2 distinct objects, because the I/O thread may be reading
+     events in a different format from what the SQL thread is reading (consider
+     the case of a master which has been upgraded from 5.0 to 5.1 without doing
+     RESET MASTER, or from 4.x to 5.0).
+  */
+  Format_description_log_event *description_event_for_exec,
+    *description_event_for_queue;
+
   void reset_bytes_written()
   {
     bytes_written = 0;
@@ -144,7 +164,8 @@ public:
   bool open(const char *log_name,enum_log_type log_type,
 	    const char *new_name, const char *index_file_name_arg,
 	    enum cache_type io_cache_type_arg,
-	    bool no_auto_events_arg, ulong max_size);
+	    bool no_auto_events_arg, ulong max_size,
+            bool null_created);
   void new_file(bool need_lock= 1);
   bool write(THD *thd, enum enum_server_command command,
 	     const char *format,...);
@@ -590,9 +611,10 @@ public:
      the connection
     priv_user - The user privilege we are using. May be '' for anonymous user.
     db - currently selected database
+    catalog - currently selected catalog
     ip - client IP
    */
-  char	  *host,*user,*priv_user,*db,*ip;
+  char	  *host,*user,*priv_user,*db,*catalog,*ip;
   char	  priv_host[MAX_HOSTNAME];
   /* remote (peer) port */
   uint16 peer_port;