unix: use fcntl(2) libc stub on OpenBSD

[jrick]

Future OpenBSD releases will remove the syscall(2) interface.
This converts the Fcntl functions to use fcntl(2) from libc,
rather than indirectly through syscall(2).

Updates golang/go#63900.

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Patch Set 1:

(1 comment)

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Message from Josh Rickmar:

Patch Set 3:

(1 comment)

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Message from Bryan Mills:

Patch Set 4: Code-Review+1

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Message from Mauri de Souza Meneguzzo:

Patch Set 5: Code-Review+1

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Message from Joel Sing:

Patch Set 5: Code-Review+2 Run-TryBot+1

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Patch Set 5:

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Message from Gopher Robot:

Patch Set 5: TryBot-Result+1

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Message from Cherry Mui:

Patch Set 5: Code-Review+1

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This PR is being closed because golang.org/cl/538995 has been merged.