About set_thread_res_uid and Uid types

[rusty-snake]

Uid in rustix has a strange history of not liking the value -1. While it is nowadays sound (no unsafe) to create one, it is still incorrect.

At the same time the set_thread_res_uid takes three Uids that have a clear use of being -1 in this syscall interface.

It is possible to implement set_thread_euid like

set_thread_res_uid(
    Uid::from_raw_unchecked(-1_i32 as u32),
    euid,
    Uid::from_raw_unchecked(-1_i32 as u32),
)?;

this is

• ugly, complicated and long
• documented as being incorrect

So my suggestion is to change set_thread_res_uid to take three Option<Uid> in a semver compatible way.