rust: library: Add setsid method to CommandExt trait

[LevitatingBusinessMan]

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: #105376

ACP: rust-lang/libs-team#184

This PR was previously submitted by @HarveyHunt (whom I marked as Co-Author in the commit message) in #105377. However that PR went stale.

I applied the suggestion to change the function signature to fn setsid(&mut self, setsid: bool) -> &mut Command.

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[LevitatingBusinessMan]

@rustbot label +T-libs-api -T-libs

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  • 8285fe4

[rust-log-analyzer]

The job mingw-check-1 failed! Check out the build log: (web) (plain)

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[RUSTC-TIMING] object test:false 10.734
error[E0425]: cannot find value `POSIX_SPAWN_SETSID` in crate `libc`
    --> library/std/src/sys/process/unix/unix.rs:750:32
     |
750  |                 flags |= libc::POSIX_SPAWN_SETSID;
     |                                ^^^^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `POSIX_SPAWN_SETSIGDEF`
     |
    ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libc-0.2.172/src/unix/bsd/apple/mod.rs:5232:1
     |
5232 | pub const POSIX_SPAWN_SETSIGDEF: c_int = 0x0004;
     | -------------------------------------- similarly named constant `POSIX_SPAWN_SETSIGDEF` defined here