Default of max_stack_depth and getrlimit

In 8.2, we started to use getrlimit(RLIMIT_STACK, ...) to set 
max_stack_depth automatically, to a max of 2MB:

http://archives.postgresql.org/pgsql-hackers/2006-10/msg00389.php

However, it's not behaving as I expected when the stack limit is set to 
"unlimited". I would expect max_stack_depth to be set to the max of 2MB, 
but instead it gets set to 100kB.

I don't normally run without a limit, but it looks like the regression 
tests run like that with "make check", at least on my platform. I bumped 
into this while running a custom regression test with very deep nesting.

I think we should differentiate between "infinite" and "unknown" in the 
return value of get_stack_depth_limit(), and use max_stack_depth of 2MB 
in case of infinite, and fall back to the 100kB only in the unknown case.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com