`openpty` returns `RawFd` for master rather than `PtyMaster`

[yilinwei]

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I am not too familiar with the API, but I don't think there's any reason it shouldn't return a PtyMaster instead; it means that the result cannot be used in grantpt etc...

[asomers]

@Susurrus what do you think?

[daniel-brenot]

Bumping this because it's an issue for me as well...