Fix some issues in libthread_xu's condvar implementation.

Fix some issues in libthread_xu's condvar implementation.

* Non-broadcast mode is not guaranteed to signal just one waiter, loosen
  it up a bit to close race conditions and signal more if necessary.

* Clean up the condition structure.  Do not try to track non-broadcast
  wakeups.  Do not try to block waiting for individual wakeups... the
  spec does not require it and, in fact, doing so can create more
  problems then it solves.

* Load oldseq from cv->c_seqno *BEFORE* releasing the passed mutex to
  close a race.  The mutex is there precisely so that userland can
  guarantee that no race will occur between waiter and signaler.

Reported-by: Jordan Gordeev <jgordeev@dir.bg>,
             "Simon 'corecode' Schubert" <corecode@xxxxxxxxxxxx>