The `libc::sigaction` struct doesn't contain any way to access the underlying `sa_handler` field.

Per https://pubs.opengroup.org/onlinepubs/007904875/functions/sigaction.html and https://man7.org/linux/man-pages/man2/sigaction.2.html, there's an sa_handler field for the sigaction struct. On Linux with some architectures, it may be aliased to sa_sigaction, but it's always there.

This is required in order to know what the previous signal handler was, and more importantly, whether it was previously ignored (.sa_handler == SIG_IGN) or set to the default disposition (.sa_handler == SIG_DFL).

Also, this allows omitting the SA_SIGINFO property of sa_flags, offering a simpler API for most common cases as it's just one (commonly-ignored) argument, not three.

Start by comparing the platform-specific libc::sigaction definitions with the POSIX and Linux sigaction documentation linked in the issue, paying attention to architectures where sa_handler is aliased to sa_sigaction. The work is done when callers can inspect the previous handler, including SIG_IGN and SIG_DFL, without requiring SA_SIGINFO.