thread and proc have been copied and zeroed from the old thread and
proc. Otherwise attempts to modify thread or process data in sched_fork()
could be undone.
- Don't copy td_{base,}_user_pri from the old thread to the new thread in
sched_fork_thread() in ULE. This is already done courtesy the bcopy()
of the thread copy region.
- Always initialize the real priority (td_priority) of new threads to the
new thread's base priority (td_base_pri) to avoid bogusly inheriting a
borrowed priority from the parent thread.
MFC after: 2 weeks
22d19207e9