Change the logic for the default for MaxMemInQueues

If we can't detect the physical memory, the new default is 8 GB on
64-bit architectures, and 1 GB on 32-bit architectures.

If we *can* detect the physical memory, the new default is
  CLAMP(256 MB, phys_mem * 0.75, MAX_DFLT)
where MAX_DFLT is 8 GB on 64-bit architectures and 2 GB on 32-bit
architectures.

You can still override the default by hand.  The logic here is simply
trying to choose a lower default value on systems with less than 12 GB
of physical RAM.