Allow both &[CString] and &[&CStr] for sequence args in unistd

[youknowone]

I am new to this project, so I wonder this kind of change is acceptable for the project or not.

I agree taking &[&CStr] is the ideal way for the functions. But unfortunately, when users generate CStrings from &str, they may get &[CString] rather than &[&CStr]
Unlike single CString to &CStr argument, there is no easy way to convert &[CString] to &[&CStr] without creating new vector.

By changing the type from &[&CStr] to &[AsRef<CStr>], the args now can take both &[CString] and &[&CStr] without changing that much from nix.

[asomers]

Yes, that sounds like a good change. Could you please add a CHANGELOG entry? We also require tests for all new functionality. These functions already have some tests in test/test_unistd.rs. But it might be more appropriate in this case to add some doc tests.

[youknowone]

I fixed test_unistd.rs to check both types of input

[youknowone]

@asomers would you review this PR again?

[asomers]

bors r+

[bors]

Build succeeded:

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