Fix system dependent signal handling logic

[hibiken]

Closes #129

[codecov]

Codecov Report

Merging #130 into master will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   81.10%   81.05%   -0.05%     
==========================================
  Files          14       15       +1     
  Lines        1344     1346       +2     
==========================================
+ Hits         1090     1091       +1     
- Misses        174      175       +1     
  Partials       80       80              

Impacted Files	Coverage Δ
background.go	67.05% <0.00%> (+5.76%)	⬆️
signals_unix.go	0.00% <0.00%> (ø)
processor.go	79.61% <0.00%> (+0.48%)	⬆️

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