refactor: redefine ForkptyResult to avoid uninited master field

[SteveLauC]

What does this PR do

Before this PR, our ForkptrResult was defined like this:

pub struct ForkptyResult {
    pub master: OwnedFd,
    pub fork_result: ForkResult,
}

The problem with this definition is that the master field won't be initialized (actually, it won't be accessed at all) within the forked child process, which means that our forkpty() was calling OwnedFd::from_raw_fd(an uninitialized memory), if the dirty value stored in that uninitialized memory is not a valid value for a file descriptor (say -1), then this function will panic, making the child process panic.

This PR redefines the ForkptyResult type so that the master field is only present in the parent process.

cc @Xeom

related to #2312

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[Xeom]

This looks perfect - thank you so much!

I've tried out your branch in the original codebase where I was experiencing a crash, and it works great.

[asomers]

Yes, I think this is exactly right.