MFCr350612

MFCr350612
  procdesc: fix reparenting when the debugger is attached

  The process is reparented to the debugger while it is attached.
    B          B
   /   ---->   |
  A          A D

  Every time when the process is reparented, it is added to the orphan list
  of the previous parent:

  A->orphan = B
  D->orphan = NULL

  When the A process will close the process descriptor to the B process,
  the B process will be reparented to the init process.
    B            B - init
    |   ---->
  A D          A   D

  A->orphan = B
  D->orphan = B

  In this scenario, the B process is in the orphan list of A and D.

  When the last process descriptor is closed instead of reparenting
  it to the reaper let it stay with the debugger process and set
  our previews parent to the reaper.

  Add test case for this situation.
  Notice that without this patch the kernel will crash with this test case:
  panic: orphan 0xfffff8000e990530 of 0xfffff8000e990000 has unexpected oppid 1

  Reviewed by:  markj, kib
  Differential Revision:        https://reviews.freebsd.org/D20361