MFC r327560:

MFC r327560:

  Allow timed waits with relative timeouts on locks and condvars.

  Even though pthreads doesn't support this, there are various alternative
  APIs that use this. For example, uv_cond_timedwait() accepts a relative
  timeout. So does Rust's std::sync::Condvar::wait_timeout().

  Though I personally think that relative timeouts are bad (due to
  imprecision for repeated operations), it does seem that people want
  this. Extend the existing futex functions to keep track of whether an
  absolute timeout is used in a boolean flag.