Backport from mysql-5.5 to mysql-5.1 of:

Bug19770858: MYSQLD CAN BE DRIVEN TO OOM WITH TWO SIMPLE SESSION VARS

The problem was that the maximum value of the transaction_prealloc_size
session system variable was ULONG_MAX which meant that it was possible
to cause the server to allocate excessive amounts of memory.

This patch fixes the problem by reducing the maxmimum value of
transaction_prealloc_size and transaction_alloc_block_size down
to 128K.

Note that transactions will still be able to allocate more than
128K if needed, this patch just reduces the amount that can be
preallocated - as well as the maximum size of the incremental
allocation blocks.

(cherry picked from commit 540c9f7ebb428bbf9ec028feabe1f7f919fdefd9)

Conflicts:
	mysql-test/suite/sys_vars/r/transaction_alloc_block_size_basic.result
	mysql-test/suite/sys_vars/r/transaction_alloc_block_size_basic_64.result
	mysql-test/suite/sys_vars/t/disabled.def
	mysql-test/suite/sys_vars/t/transaction_alloc_block_size_basic.test
	sql/sys_vars.cc