unix: add unix.TimeToPtpClockTime on Linux

[yrk-lab]

Add time conversion helpers for PtpClockTime similar to those
existing for Timespec and Timeval.

Allows to use the maths of the standard time package while avoiding
the boilerplate:
write "ptpt.Time()" instead of "time.Unix(ptpt.Sec, int64(ptpt.Nsec))",
and "unix.TimeToPtpClockTime(t)" instead of
"unix.PtpClockTime{Sec: t.Unix(), Nsec: uint32(t.Nanosecond())}".

Fixes golang/go#70032

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Patch Set 1:

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Patch Set 1:

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Patch Set 1: Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-10-22T22:45:28Z","revision":"d5b6ecbfb75bb555c21c2565098eaf833d8b06ab"}

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Patch Set 1: -Commit-Queue

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This CL has passed the run

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Patch Set 1: LUCI-TryBot-Result+1

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Patch Set 1:

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Message from Yaroslav Kolomiiets:

Patch Set 2:

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Patch Set 2:

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Patch Set 2:

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Patch Set 3: Hold+1

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