pidwait: replace busy-loop with pidfd_open and poll on Linux

[Krysztal112233]

This PR pick the Linux implementation from PR #400 to replace the inefficient busy-loop polling (/proc every 50ms) with pidfd_open + poll, which blocks efficiently in the kernel with zero CPU usage.

Also added a simple test to verify the wait works correctly.

Thanks for @FurryR

[codecov]

Codecov Report

❌ Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.19%. Comparing base (a995b90) to head (8df52cb).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/pidwait/src/wait.rs	0.00%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #711      +/-   ##
==========================================
- Coverage   19.21%   19.19%   -0.02%     
==========================================
  Files          68       68              
  Lines       10150    10158       +8     
  Branches      546      546              
==========================================
  Hits         1950     1950              
- Misses       8200     8208       +8     

Flag	Coverage Δ
macos_latest	6.74% <ø> (ø)
ubuntu_latest	19.55% <0.00%> (-0.02%)	⬇️
windows_latest	0.02% <ø> (ø)

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[FurryR]

Nice to see the new implementation here. Ima like dropping my pull request
Will see if I can help.