kernel - Be nicer to pthreads in vfork()

kernel - Be nicer to pthreads in vfork()

* When vfork()ing, give the new sub-process's lwp the same TID as the one
  that called vfork().  Even though user processes are not supposed to do
  anything sophisticated inside a vfork() prior to exec()ing, some things
  such as fileno() having to lock in a threaded environment might not be
  apparent to the programmer.

* By giving the sub-process the same TID, operations done inside the
  vfork() prior to exec that interact with pthreads will not confuse
  pthreads and cause corruption due to e.g. TID 0 clashing with TID 0
  running in the parent that is running concurrently.