Ignore SIGCHLD to prevent creating zombies

[oxalica]

As #200 (comment) pointed out, calling waitpid once on SIGCHLD is not enough. Multiple children may died during the delivery of the signal. I also hit this in production with earlyoom 1.7.

This PR uses signal(SIGCHLD, SIG_IGN); to prevents creations of zombies.
In manual sigaction(2),

POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN.
POSIX.1-2001 and later allow this possibility, so that ignoring SIGCHLD
can be used to prevent the creation of zombies (see wait(2)).

I think relying on POSIX 2001 should be enough for us. But note that ignoring SIGCHLD seems also prevent any usage of wait* functions. Currently we doesn't do it. Alternatively, we could run a loop in the signal handler to achieve the same goal.

Here is the test which proves a single waitpid is not enough,

#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>
#include <spawn.h>

void handler(int _sig) {
    waitpid(-1, NULL, WNOHANG);
}

int main() {
    /* This still makes some zombies. The actual number of zombies may varies. */
    signal(SIGCHLD, handler);
    /* This creates no zombies always. */
    /* signal(SIGCHLD, SIG_IGN); */

    char *args[] = { "true", NULL };
    for (int i = 0; i < 128; ++i)
        posix_spawnp(NULL, args[0], NULL, NULL, args, NULL);

    puts("spawned");
    for(;;)
        sleep(1);
}

[rfjakob]

❤️