Make a new sentinel variable for worker thread termination

[cottsay]

The original approach here where the signal sentinel is re-used to broadcast termination to worker threads assumed that sig_atomic_t was thread safe, which is not correct. Rather than change sig_atomic_t to a type which is both thread-safe and can be set by signal-safe mechanisms, this change breaks the two apart and introduces a sentinel specifically for broadcasting termination signals to and from worker threads.

This closes a data race identified by tsan.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.54%. Comparing base (99a71b1) to head (d8a268b).

Files with missing lines	Patch %	Lines
src/createrepo-agent/command.c	57.14%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   55.47%   55.54%   +0.06%     
==========================================
  Files          22       22              
  Lines        2830     2832       +2     
  Branches      601      601              
==========================================
+ Hits         1570     1573       +3     
+ Misses        937      936       -1     
  Partials      323      323              

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