Improve nodejs stack size limit heuristic.
Improve nodejs stack size limit heuristic.

Setting stack size exactly to the system limit will cause node to
segfault before it has a chance to notice that it ran out of space. In
my experiemnts values closer than 64 KiB may still cause a segfault from
time to time.

I also changed the multiplier to 1024, since that's what v8 actually
uses for the flag value.

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