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/mysql_priv.h

@@ -14,6 +14,15 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
+/*
+  Mostly this file is used in the server. But a little part of it is used in
+  mysqlbinlog too (definition of SELECT_DISTINCT and others).
+  The consequence is that 90% of the file is wrapped in #ifndef MYSQL_CLIENT,
+  except the part which must be in the server and in the client.
+*/
+
+#ifndef MYSQL_CLIENT
+
 #include <my_global.h>
 #include <assert.h>
 #include <mysql_version.h>
@@ -176,7 +185,15 @@ extern CHARSET_INFO *national_charset_info, *table_alias_charset;
 #define TEST_NO_STACKTRACE	512
 #define TEST_SIGINT		1024	/* Allow sigint on threads */
 
-/* options for select set by the yacc parser (stored in lex->options) */
+#endif
+
+/* 
+   This is included in the server and in the client.
+   Options for select set by the yacc parser (stored in lex->options).
+   None of the 32 defines below should have its value changed, or this will
+   break replication.
+*/
+
 #define SELECT_DISTINCT		(1L << 0)
 #define SELECT_STRAIGHT_JOIN	(1L << 1)
 #define SELECT_DESCRIBE		(1L << 2)
@@ -214,6 +231,9 @@ extern CHARSET_INFO *national_charset_info, *table_alias_charset;
 #define OPTION_RELAXED_UNIQUE_CHECKS    (1L << 27)
 #define SELECT_NO_UNLOCK                (1L << 28)
 
+/* The rest of the file is included in the server only */
+#ifndef MYSQL_CLIENT
+
 /* options for UNION set by the yacc parser (stored in unit->union_option) */
 #define UNION_ALL               1
 
@@ -1102,3 +1122,5 @@ inline void setup_table_map(TABLE *table, TABLE_LIST *table_list, uint tablenr)
   table->map= (table_map) 1 << tablenr;
   table->force_index= table_list->force_index;
 }
+
+#endif /* MYSQL_CLIENT */