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.)