Prevent orphan child processes when CBMC parent is killed

[tautschnig]

Add prctl(PR_SET_PDEATHSIG, SIGTERM) in the child process fork path so the kernel sends SIGTERM to the child (e.g., z3) when the parent dies. This prevents orphan processes from persisting indefinitely after CBMC is killed with SIGKILL.

The prctl call is guarded by #ifdef linux since it is Linux-specific.

Fixes: #8485

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.41%. Comparing base (59d2211) to head (34dc275).

Files with missing lines	Patch %	Lines
src/util/piped_process.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8900      +/-   ##
===========================================
- Coverage    80.41%   80.41%   -0.01%     
===========================================
  Files         1703     1703              
  Lines       188398   188399       +1     
  Branches        73       73              
===========================================
- Hits        151502   151499       -3     
- Misses       36896    36900       +4     

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