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.