macos: restoring signal disposition doesn't work as flags are not propagated

Take this very basic example:

#include <stdio.h>
#include <signal.h>
#include <stdlib.h>

void handler(int signum) {
    printf("Signal handler called with signal %d\n", signum);
}

int main() {
    struct sigaction sa, old_sa;

    // Set a signal handler with SA_RESETHAND flag
    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESETHAND;
    if (sigaction(SIGINT, &sa, NULL) == -1) {
        perror("sigaction");
        exit(1);
    }

    // Set a second signal handler without SA_RESETHAND flag
    sa.sa_flags = 0;
    if (sigaction(SIGINT, &sa, &old_sa) == -1) {
        perror("sigaction");
        exit(1);
    }

    // Check whether the SA_RESETHAND flag is actually propagated
    if (old_sa.sa_flags & SA_RESETHAND) {
        printf("SA_RESETHAND was set in the old signal handler\n");
    } else {
        printf("SA_RESETHAND was not set in the old signal handler\n");
    }

    return 0;
}

Running this on linux produces the correct output:

$ SA_RESETHAND was set in the old signal handler

Whereas on macos (arm64) it doesn't:

$ SA_RESETHAND was not set in the old signal handler

I observed this while running current v1.x with the node.js test suite and caused some signal tests to fail. They started failing after https://github.com/libuv/libuv/commit/b9421d70665352138557d2d2338656a38ac70691, though I don't know if there's much we can do from the libuv side as it seems a macos issue. Thoughts?

Start with the C reproducer in the issue and compare macOS and Linux sigaction behavior, then review the libuv change referenced by commit b9421d70665352138557d2d2338656a38ac70691. The issue names no repository file or test path; done means determining whether libuv can work around the macOS flag behavior or documenting that it cannot, with the Node.js signal failures understood.