kernel - Fix p->p_lock race and remove unused procedures

kernel - Fix p->p_lock race and remove unused procedures

* Remove vm_waitproc() (move its vmspace_exitfree() call to the one place
  that calls vm_waitproc()).

* Remove cpu_proc_wait(), which only called pmap_dispose_proc().

* Remove pmap_dispose_proc(), which only had an (incorrect) assertion in it
  and otherwise did nothing.

* Wait for the p->p_lock count to drop to zero before calling
  vmspace_exitfree(), and then wait again afterwords now that there
  are absolutely no possible references to (p) left.