refactor(unix_signal): provide a conservative table size

[cnjhb]

Because the signal values ​​are not consecutive, a conservative size is used to avoid wasting memory. However, even this conservative size reduces the number of table expansions.

Modern compilers are smart enough that the initial statistics should not incur any performance overhead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (39143f0) to head (0f4c7cb).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4069      +/-   ##
==========================================
+ Coverage   90.44%   90.46%   +0.01%     
==========================================
  Files         939      940       +1     
  Lines       60346    60348       +2     
  Branches     1145     1145              
==========================================
+ Hits        54582    54592      +10     
+ Misses       5254     5249       -5     
+ Partials      510      507       -3     

Files with missing lines	Coverage Δ
luaa.c	65.40% <100.00%> (-2.84%)	⬇️
unix_signals.h	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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