finish amd64 port of DragonFly GSoC project

[Jordan Gordeev]

Matthew Dillon wrote:
    It's looking very good so far.  After I recompiled the world the
    segmentation faults went away.  It looks like you implemented the
    entire 128TB user VM space and a 512G kernel VM space in the new
    amd64 pmap code?
  
The constants in sys/platform/pc64/include/pmap.h indicate that 128TB is 
available for user VM space and at most 512GB is available for the 
kernel, but I'm not sure what is the actual limit for the kernel VM space