feat: support to build in Darwin

[Yu-Jack]

Which issue(s) this PR fixes:

longhorn/longhorn#9487

What this PR does / why we need it:

Like longhorn/longhorn#9487 mentioned, it needs to support to build in Darwin.

Special notes for your reviewer:

I've changed syscall to unix in this PR, but I'm not sure what is actually needed in types/namespace_darwin.go and ns/joiner_darwin.go. Therefore, I left it as a no-op.

Additional documentation or context

None.

[codecov]

Codecov Report

Attention: Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.81%. Comparing base (797b589) to head (16b0f9d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ns/joiner_linux.go	56.25%	4 Missing and 3 partials ⚠️
sys/sys.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   79.78%   79.81%   +0.02%     
==========================================
  Files          33       35       +2     
  Lines        1583     1585       +2     
==========================================
+ Hits         1263     1265       +2     
  Misses        204      204              
  Partials      116      116              

Flag	Coverage Δ
unittests	79.81% <74.19%> (+0.02%)	⬆️

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[derekbit]

LGTM

[c3y1huang]

@Yu-Jack , is this ready?

[Yu-Jack]

@Yu-Jack , is this ready for review?

@c3y1huang , I'm still checking Codecov code coverage issue. It is supposed to test joiner_linux.go. I'll mark it ready after I finish my investigation.

[Yu-Jack]

@c3y1huang It's ready. joiner linux.go had been tested actually. I just misunderstood some description ...