windows: fix filetime conversions and deprecate Nanoseconds

[gremat]

Filetime.Nanoseconds() has a major drawback: the returned int64 is too small to represent Filetime's zero value (January 1, 1601, [1]) in terms of nanoseconds since Epoch (00:00:00 UTC, January 1, 1970); MinInt64 [2] only dates back to year 1677.

This has real-life implications, e.g., some Windows sub systems (Perflib, to name one) create registry keys with the last write time property set to zero (see note below). In this case, ModTime() reports an underflow-affected value of 2185-07-22T00:34:33.709551+01:00.

This commit deprecates Nanoseconds() in favor of the new Unix() that returns a well-known pair of seconds and nanoseconds [3] thus is capable to cover the full range of Filetime values. ModTime() uses Unix() accordingly to produce correct time.Time values again. Additionally, ModTimeZero() provides a convenient way to check for a last write time value of zero in analogy to time.Time.IsZero() [4]; no need to specify January 1, 1601 manually.

A note on last write time values: lastWriteTime is not exposed in the UI (say, regedit) or in PowerShell (Get-ItemProperty), you need to query RegQueryInfoKeyA [5] explicitly in some way [6] or another [7]. The source of the latter is offline by now but can be found elsewhere [8] and provides a quick way to show the value.

[1] https://learn.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-filetime
[2] https://pkg.go.dev/math#pkg-constants
[3] https://pkg.go.dev/time#Unix
[4] https://pkg.go.dev/time#Time.IsZero
[5] https://learn.microsoft.com/en-us/windows/win32/api/winreg/nf-winreg-regqueryinfokeya?redirectedfrom=MSDN
[6] https://learn.microsoft.com/en-us/windows/win32/sysinfo/retrieving-the-last-write-time
[7] https://learn-powershell.net/2014/12/18/retrieving-a-registry-key-lastwritetime-using-powershell/
[8] https://github.com/wxrdnx/GetRegistryKeyLastWriteTimeAndClassName

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