This patch allows you to set stacksize=0 in the config
file to specify OS manged stack size.
If stacksize is set to zero or less (it has to be set,
it can't be left out), no stack size is specified when
creating threads, and in the case of Linux at least,
the stack will start off about 4k and grow automatically.
In all other respects behaviour is the same as before.
i.e. if you do not specify anything, stacksize
defaults to 64k; if you specify something more than
zero but less then 16k, it's rounded up to 16k.