kernel - Change lwp_fork() to not hold the mplock in the new thread

kernel - Change lwp_fork() to not hold the mplock in the new thread

* Change lwp_fork() to produce a mpsafe thread at startup instead of
  one with the mplock held.

* Change all fork_trampoline() functions and all kernel callbacks
  via cpu_set_fork_handler() to expect a thread without the
  mplock held.

* Adjust the thread procedures for aio etc (those not yet mpsafe)
  to acquire the mplock.