Change: the default JS stack size is reduced to 64k.

Change: the default JS stack size is reduced to 64k.

After 86784a68e8c8 (Computed goto) the size of a stack frame
njs_vmcode_interpreter() when -O0 is specified during compilation
increased significantly.  This might cause system stack overflow for very
deep recursive calls because system stack is exausted faster than JS
stack.

It is possible now to specify JS stack size for CLI.