Support PreserveTimes for symlinks on Unix

[fako1024]

This is an addition to #31 - While symlink timestamps cannot be set universally (neither at time of link creation nor by a simple call to os.Chtimes()) they can be set on Unix making use of unix.Lutimes(). This PR extends the existing PreserveTime option by that, transferring the original link timestamp(s) to the newly created one if the option is set.

For unsupported architectures, a stub is provided similar to already existing ones (e.g. for ownership preservation).

[codecov-commenter]

Codecov Report

Merging #83 (55416e9) into main (be95711) will increase coverage by 1.43%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   77.40%   78.83%   +1.43%     
==========================================
  Files          13       15       +2     
  Lines         177      189      +12     
==========================================
+ Hits          137      149      +12     
  Misses         20       20              
  Partials       20       20              

Impacted Files	Coverage Δ
copy.go	76.47% <100.00%> (+0.96%)	⬆️
preserve_ltimes.go	100.00% <100.00%> (ø)
preserve_ltimes_x.go	100.00% <100.00%> (ø)

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[otiai10]

Getting Lstat in onsymlink could be inefficient operation for platforms who don't even need it.

[fako1024]

@otiai10 It seems that a recent upstream change breaks the GopherJS Pipeline (unrelated to this issue):
gopherjs/gopherjs@f754ef8

What's your preference, should I open an independent issue for that? Do you want to fix it on main directly for a rebase?

[otiai10]

Thank you, @fako1024

open an independent issue for that

This would be super cool! Thank you so much ;)

[otiai10]

Thank you for everything!