Change typedefs for vm_offset_t, vm_size_t, and vm_pindex_t, add szmin/sz/max

Change typedefs for vm_offset_t, vm_size_t, and vm_pindex_t, add szmin/sz/max

* Generally change these types from unsigned int to unsigned long in
  order to better catch int/long issues on both 32 and 64 bit builds.

* Add szmin() and szmax() inlines which take and return size_t.