Block SIGCHLD so we don't get polluted by system() in another thread[1]

Block SIGCHLD so we don't get polluted by system() in another thread[1]
(though this seem to only happen with debuggers and (?)broken OSes)

[1] system() blocks SIGCHLD, and is probably not intended for multithreading

We cannot assume that blocking calls in other libraries ignore EINTR anyway,
so the only safe approach is to block in all threads (you can unblock it in
your thread if you really want it) except the signal handling thread.