Revision 184172

Close races in the pthread_condvar implementation in 6.x, note that
7.x and beyond have very different implementations and these fixes
do not apply.

In the kernel:

  Move the flag UQF_UMTXQ from the thread's td_flags where it was
  unlocked to the umtx_q struct where it now has locking.

  Nuke the UMTX_DYNAMIC_SHARED define while here, it's unused and
  complex.

  Refactor do_wait so that if we get a signal or timeout AND woken
  up, we forward the wakeup to any other waiters pending to avoid
  a missed wakeup.

In userland:

  Bring over some fixes from DragonFlyBSD for the userland part of
  pthread_condvars, basically relax the consistency on the number of
  waiters and signals pending because it was both too strict and
  incorrect causing lost wakeups.

Reviewed by: davidxu
Approved by: re