Increase stack size on macOS as well as Linux

The code works fine on macOS, but the default stack size we attempt to
set is larger than what my system will allow (Nix attempts to set the
stack size to 67108864, but the maximum allowed is 67092480), so I've
instead used the requested stack size or the maximum allowed, whichever
is smaller.

I've also added an error message if setting the stack size fails. It
looks like this:

> Failed to increase stack size from 8372224 to 67108864 (maximum
> allowed stack size: 67092480): Invalid argument

src/libutil/current-process.hh

@@ -16,7 +16,7 @@ unsigned int getMaxCPU();
 /**
  * Change the stack size.
  */
-void setStackSize(size_t stackSize);
+void setStackSize(rlim_t stackSize);
 
 /**
  * Restore the original inherited Unix process context (such as signal