Restore ProcessPool signal dispositions after start

[binaryfire]

Process\Pool::start() temporarily replaces its control-signal handlers. It restored only the previous handler pointer through Swoole's callback registration path, so signals that originally used SIG_DFL remained routed through an empty Swoole callback after the pool stopped.

Save and restore each complete signal action, and clear the temporary Swoole registry entry before restoring it. This preserves the original handler, flags, and mask.

The regression runs the pool in an isolated child and verifies that Linux SIGIO returns to its default terminating behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.24%. Comparing base (07606e5) to head (321ef49).

Additional details and impacted files

@@           Coverage Diff           @@
##              6.1    #6230   +/-   ##
=======================================
  Coverage   86.24%   86.24%           
=======================================
  Files         110      110           
  Lines       16790    16790           
  Branches     2980     2980           
=======================================
  Hits        14481    14481           
  Misses       2309     2309           

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