We want our default stack size limit to be approximately 2MB, to be safe for
JS helper tasks that can use a lot of stack, but expect most threads to use
much less. On Linux, however, requesting a stack of 2MB or larger risks the
kernel allocating an entire 2MB huge page for it on first access, which we do
not want. To avoid this possibility, we subtract 2 standard VM page sizes
from our default.