[sairex53]

Fixes the fork-to-exec signal race reported in #566. Signals remain blocked across the command fork; the child restores the saved signal dispositions before restoring its signal mask, so a forwarded signal stays pending until the command can receive it. This also preserves signals that were ignored by sudo\x27s caller.

Thanks to @pepa-cz for the detailed analysis and reproducer in #566; this implements the approach described there.

Validation on Fedora 44 aarch64 (kernel 7.2.6), building in an Ubuntu 24.04 arm64 container:

• make check passes.
• The issue reproducer terminated the command in 50/50 runs with and without use_pty (0 lost SIGTERMs).
• Verified an ignored SIGTERM remains ignored by the executed command.