Fix SIGCHLD handling

[kxxt]

Using ptrace::cont will skip some or a lot of syscalls depending on scenario. ptrace::syscall should be used here.

Reproducer:

#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>

int main() {
    if (0 == fork()) {
        exit(0);
    } else {
        wait(NULL);
        fork();
    }
    return 0;
}
// gcc reproducer.c -o reproducer

To reproduce, run cargo run -- ./reproducer and compare the output with or without this PR. Without this PR, the final clone syscall doesn't show.

[283807] munmap(0x7FFFF7F63000, 379247) = 0
[283807] clone(0x1200011, 0) = 0x454A0
[283807] wait4(4294967295, 0, 0, 0x0) = 0x454A0
[283807] clone(0x1200011, 0) = 0x454A1 # This line only shows with this PR
[283807] exit_group(0) = ?

Actually I fixed a similar bug in tracexec, which is a project that takes some inspiration from lurk, but I forgot to contribute it back to lurk at that time.

Fixes #32