Add capability for pipe_notify() to raise a signal

[shikokuchuo]

Provides solution for shikokuchuo/mirai#87.

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (cfc964f) 87.14% compared to head (8ff4b23) 86.78%.
Report is 1 commits behind head on main.

❗ Current head 8ff4b23 differs from pull request most recent head 6a9d328. Consider uploading reports for the commit 6a9d328 to get more accurate results

Files	Patch %	Lines
src/aio.c	78.04%	9 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   87.14%   86.78%   -0.36%     
==========================================
  Files          22       22              
  Lines        3500     3504       +4     
==========================================
- Hits         3050     3041       -9     
- Misses        450      463      +13     

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