The STACK_MIN_SIZE is currently set to 8192, when we actually need 
(emperically discovered) 9236 bytes to raise an fatal error, on Ubuntu 
Dapper Drake, libc6 2.3.6-0ubuntu2, Linux kernel 2.6.15-27-686, on x86.

I'm taking that as a new lower bound, plus 100B of wiggle-room for sundry
word sizes and stack behaviors.

The added test verifies in a cross-platform way that there are no gaps 
between the space that we think we need and what we actually need to report 
an error.

DOCUMENTERS:  This also adds "let" to the mysqltest commands that evaluate
an argument to expand variables therein.  (Only right of the "=", of course.)


client/mysqltest.c:
  Add "let" to the list of mysqltest commands that evaluates its arguments (only
  the right-hand-side of the equals-sign expression).
mysql-test/r/mysqltest.result:
  Added test to show that mysqltest "let" will evaluate the RHS correctly (and 
  expand the backslash test).
mysql-test/t/mysqltest.test:
  Added test to show that mysqltest "let" will evaluate the RHS correctly (and 
  expand the backslash test).
sql/mysql_priv.h:
  Increase the amount we require to be available for the stack, since 
  experience told us that the previous amount was too little by at least
  1044 bytes.
mysql-test/r/execution_constants.result:
  New BitKeeper file ``mysql-test/r/execution_constants.result''
mysql-test/t/execution_constants.test:
  New BitKeeper file ``mysql-test/t/execution_constants.test''