Major overfaul of futex implemention:
- Use sleepqs directly, rather than using condition variables and
  separate wait queues / strutures.  By doing this, and using the
  standard mechanism for keeping sleepqs sorted by priority, we
  acn ensure that the highest priority waiters will be awakened,
  rather than naively awakening in FIFO order.
- As part of the data structure re-organization, struct lwp gains
  "l_futex" (the futex an LWP is blocked on) and "l_futex_wakesel"
  (the futex wake selector bitset) fields (and loses l___rsvd1).
  Plese note the special locking considerations for these fields
  documented in the comments.
- Add the notion of a "futex class".  This is prep work for eventually
  supporting the FUTEX_*_PI operations, as well as some future NetBSD
  extensions to the futex interface.
- Add a preliminary implementation of the first of those NetBSD extensions,
  FUTEX_NETBSD_RW_WAIT and FUTEX_NETBSD_RW_HANDOFF.  These are designed
  to implement reader/writer locks with direct-handoff to the correct
  priority thread(s) (real-time read-waiters need to have priority over
  non-real-time write-waiters).  NOTE: this is currently disabled due to
  a mysterious panic that haasn't yet been tracked down.
- Add some SDT probes to aid in debugging.

(thorpej)