Today, all threads waiting for a given futex are woken in FIFO order (first
waiter woken first) instead of priority order.

This patch makes use of plist (pirotity ordered lists) instead of simple list in
futex_hash_bucket.

All non-RT threads are stored with priority MAX_RT_PRIO, causing them to be
woken last, in FIFO order (RT-threads are woken first, in priority order).

Signed-off-by: S