Propagate signal exit status to parent process

[magicant]

POSIX.1-2024 updated the requirements on the exit built-in. We now examine the exit status of the last command and if it is a signal number we propagate the signal to the shell process.

See magicant/yash-rs#491 for the yash-rs equivalent.

Summary by Copilot

This pull request introduces a significant change to how the shell handles exit statuses when commands are terminated by signals. It ensures that the shell propagates the termination signal of the last command by terminating itself with the same signal. Additionally, it updates documentation and tests to reflect this behavior and introduces a new helper function in the codebase.

Changes to exit status handling:

• Shell termination with signals: The shell now terminates itself with the same signal as the last command if the command's exit status indicates termination by a signal. This behavior is implemented in the exit_shell_with_status function, with a new helper function maybe_raise to handle signal propagation. (yash.c, [1] [2]

Documentation updates:

• English documentation: Updated the doc/_exit.txt and doc/_return.txt files to explain the new behavior for exit statuses greater than 384 and clarify that such values are non-portable extensions. [1] [2] [3] [4]
• Japanese documentation: Synchronized updates to the Japanese documentation in doc/ja/_exit.txt, doc/ja/_return.txt, and doc/ja/exec.txt to reflect the changes in exit status handling. [1] [2] [3] [4] [5]

Test suite enhancements:

• New test cases: Added extensive test cases in tests/exit-p.tst to verify the shell's behavior when handling exit statuses greater than 256, including propagation of termination signals such as SIGTERM and SIGKILL.
• Test configuration: Updated tests/POSIX to include references to the new test file exit-p.tst.

Codebase updates:

• Helper function for signal handling: Introduced the maybe_raise function in yash.c to encapsulate the logic for terminating the shell with a signal, including special handling for non-fatal signals and disabling core dumps. [1] [2]
• Header adjustments: Added necessary includes for signal handling, such as <signal.h> and <sys/resource.h>.

These changes improve the shell's compliance with signal propagation expectations and enhance its robustness in handling termination scenarios.