rust: library: Add `setsid` method to `CommandExt` trait Add a setsid method to the CommandExt trait so that callers can create a process in a new session and process group whilst still using the POSIX spawn fast path. Tracking issue: rust-lang/rust#105376 ACP: https://github.com/rust-lang/libs-team/issues/184 This PR was previously submitted by ``@HarveyHunt`` (whom I marked as Co-Author in the commit message) in rust-lang/rust#105377. However that PR went stale. I applied the [suggestion](https://github.com/rust-lang/rust/pull/105377/files/231d19fcbfe155b2e85116865adae4253380ff1f#r1893457943) to change the function signature to `fn setsid(&mut self, setsid: bool) -> &mut Command`.