Avoid allocating in pre_exec closure

[purplesyringa]

The comment // The closure now ONLY contains async-signal-safe syscall wrappers. is misleading, since allocations are not async-signal-safe. There are three sources of allocation:

• std::io::Error::other (see Document Error::{new,other} as to be avoided in pre_exec rust-lang/rust#148971)
• format!
• CString::new

I've moved out CString::new similarly to other strings, hopefully that's idiomatic for your project.

Error::other and format! are trickier. Error messages are not transferred across pre_exec boundary, so whatever error messages you wrote were never visible. I replaced error handling with ?, which uses the non-allocating impl From<nix::Error> for std::io::Error, under assumption that if error messages weren't transmitted before, it's fine to ignore them now.

Another possible alternative would be to use .expect, effectively transmitting the error via stderr. I couldn't make a judgement on whether that's correct for use case, so I refrained from doing that.

[jamesmcm]

Thanks, good catch! (and for documenting it upstream).

This might also make a good Clippy lint.