x/sys/unix: copy ParseDirent fixes from CL 23780

[cespare]

CL 23780 fixed unsafe use of unsafe in syscall.ParseDirent (see #15653). This patch never made it to x/sys/unix.

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Side note: is there any automated process to ensure patches migrate from syscall to x/sys/unix? I had been moving all my code away from syscall, but this sort of thing makes me wonder if that's a bad idea because syscall might get more scrutiny than x/sys/unix.

[bradfitz]

Side note: is there any automated process to ensure patches migrate from syscall to x/sys/unix?

No, but with the latest progress in x/build/maintner it seems pretty easy to write a tool to find candidates that didn't get moved over. I'll write more about that tool ~next week. But on the other hand, there aren't too many such changes.

[niconegoto]

@cespare @bradfitz I can work on it.Should I wait for the automation process to be completed?

[bradfitz]

@niconegoto, no, don't wait. Any automation would only find missing work, but not do the work.

[gopherbot]

CL https://golang.org/cl/38758 mentions this issue.