Fix orderly helper appsec shutdown

[cataphract]

The sent SIGUSR1 was just terminating the process.

Description

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.40%. Comparing base (17b3c7a) to head (d63c6b3).
Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2859      +/-   ##
============================================
- Coverage     80.94%   78.40%   -2.55%     
  Complexity     2526     2526              
============================================
  Files           146      173      +27     
  Lines         14680    18709    +4029     
  Branches          0      975     +975     
============================================
+ Hits          11883    14668    +2785     
- Misses         2797     3504     +707     
- Partials          0      537     +537     

Flag	Coverage Δ
appsec-extension	69.12% <ø> (?)
tracer-extension	78.18% <ø> (ø)
tracer-php	82.10% <ø> (ø)

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see 27 files with indirect coverage changes

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