use sigaction instead of signal for SIGCHLD processing

[phirince]

The issue being handled is explained in this issue: #109

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.24 :tada:

Comparison is base (7c0d86f) 55.88% compared to head (d5ee7b0) 56.12%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #110 +/- ## ========================================== + Coverage 55.88% 56.12% +0.24% ========================================== Files 4 4 Lines 544 547 +3 Branches 145 145 ========================================== + Hits 304 307 +3 Misses 140 140 Partials 100 100 ``` | [Impacted Files](https://app.codecov.io/gh/latchset/tang/pull/110?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=latchset) | Coverage Δ | | |---|---|---| | [src/socket.c](https://app.codecov.io/gh/latchset/tang/pull/110?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=latchset#diff-c3JjL3NvY2tldC5j) | `51.78% <100.00%> (+1.32%)` | :arrow_up: |

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[sarroutbi]

@sergio-correia: can you please double-check this?