Redox: Use futex timeout to implement CondVar::wait_timeout

`CondVar::wait_timeout` is implemented by supplying a `TimeSpec` pointer to `futex`. In addition, all calls to `unimplemented!()` have been removed from the Redox `sys` module.

Related to rust-lang#45892

…chton

Redox: Return true from Path::is_absolute if a Path contains root or a scheme

In Redox, different subsystems have different filesystem paths. However, the majority of applications using the `Path::is_absolute` function really only want to know if a path is absolute from the perspective of the scheme it is currently running in, usually `file:`. This makes both `file:/` and `/` return `true` from `Path::is_absolute`, meaning that most code does not have to check if it is running on Redox.

Code that wants to know if a path contains a scheme can implement such a check on its own.

Related to rust-lang#45893

Rollup of 9 pull requests

- Successful merges: #45828, #45892, #45893, #45914, #45917, #45927, #45933, #45952, #45954
- Failed merges: