internal/socket: support MSG_DONTWAIT

[matzf]

Explicitly handle MSG_DONTWAIT in read and send calls on platforms where this is defined, to get the per-call non-blocking behavior as would be expected when calling readmsg/sendmsg in C.
When MSG_DONTWAIT is set, we always return true from the function passed to syscall.RawConn.Read/Write, to avoid entering the polling state.

Fixes golang/go#46891

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Message from Ian Lance Taylor:

Patch Set 1:

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This PR (HEAD: 4022e9b) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/net/+/333469 to see it.

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Message from Matthias Frei:

Patch Set 1:

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Message from Ian Lance Taylor:

Patch Set 2: Run-TryBot+1

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Patch Set 2:

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Patch Set 2:

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Patch Set 2: TryBot-Result-1

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Patch Set 2: Code-Review+2

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Message from Damien Neil:

Patch Set 2: Code-Review+1 Trust+1

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Message from Damien Neil:

Patch Set 2: Run-TryBot+1

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This PR is being closed because golang.org/cl/333469 has been merged.