Add OpenBSD libc syscall wrappers for vectored I/O, and share the
existing Darwin wrapper implementation and tests.

Fixes golang/go#78049

Change-Id: Ie2743ef36a5cb73b05cc551c9c1108503f5992b6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/771300
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unix package already provides support for working with file extended
attributes via Fsetxattr and Fgetxattr syscalls. Windows offers a similar
feature implemented by means of NtSetEaFile and NtQueryEaFile syscalls.
Those syscalls are provided by this patch.

Additionally, NtQueryInformationFile syscall is added. It is required
for correct implementation of extended attribute support in library
code, as correct invocation of NtQueryEaFile relies on file information
returned by said syscall. It also amends the previously accepted
NtSetInformationFile syscall.

Fixes golang/go#78672

Change-Id: Ia4ba037eb2777ba02a953a57ec1d365aad17cac5
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LUCI-TryBot-Result: [email protected] <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

This reduces some code duplication.

Change-Id: Ib648f90a26960d0bc25d2fe27e9b3fb1419ac02d
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Reviewed-by: Dmitri Shuralyov <[email protected]>
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Reviewed-by: Cherry Mui <[email protected]>
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These three feature detections have been implemented in the Go mainline.

Change-Id: I9c264f7dac0630931d851c04f904d4365d70a28b
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Change-Id: Icdd4c279693c67982f37c80a891ecadda8ec108a
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Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
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Change-Id: I2e77f6fa2ac141018c4fe5cba2c8615c9e05d99c
GitHub-Last-Rev: 5d46d8f
GitHub-Pull-Request: #274
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Reviewed-by: Junyang Shao <[email protected]>
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Reviewed-by: Dmitri Shuralyov <[email protected]>