Merge a change from MySQL to fix the failing innodb_bug34300 mysql-test:

  main.innodb_bug34300           [ fail ]
  
  mysqltest: At line 11: query 'SET @@max_allowed_packet=16777216' failed: 1621: SESSION variable 'max_allowed_packet' is read-only. Use SET GLOBAL to assign the value
  
  Aborting: main.innodb_bug34300 failed in default mode. 

The changeset is this:

  ------------------------------------------------------------
  revno: 2709.1.10
  committer: Staale Smedseng <staale.smedseng@sun.com>
  branch nick: b22891-51-bugteam
  timestamp: Thu 2008-11-20 08:51:48 +0100
  message:
    A fix for Bug#22891 "session level max_allowed_packet can be
    set but is ignored".
  
    This patch makes @@session.max_allowed_packed and
    @@session.net_buffer_length read-only as suggested in the bug
    report. The user will have to use SET GLOBAL (and reconnect)
    to alter the session values of these variables.
  
    The error string ER_VARIABLE_IS_READONLY is introduced.
  
    Tests are modified accordingly.
  modified:
    mysql-test/r/func_compress.result
    mysql-test/r/max_allowed_packet_basic.result
    mysql-test/r/max_allowed_packet_func.result
    mysql-test/r/net_buffer_length_basic.result
    mysql-test/r/packet.result
    mysql-test/r/union.result
    mysql-test/r/variables.result
    mysql-test/t/func_compress.test
    mysql-test/t/innodb_bug34300.test
    mysql-test/t/max_allowed_packet_basic.test
    mysql-test/t/max_allowed_packet_func.test
    mysql-test/t/net_buffer_length_basic.test
    mysql-test/t/packet.test
    mysql-test/t/union.test
    mysql-test/t/variables.test
    sql/set_var.cc
    sql/set_var.h
    sql/share/errmsg.txt
  ------------------------------------------------------------