`PR_SET_PDEATHSIG` is racy unless there is a check for the parent having already died

If the parent process has died already, PR_SET_PDEATHSIG is a no-op. I consider this a kernel bug, but there is a workaround: if one knows what the parent process’s PID should be, one can compare the return value of getppid() to it. If they are different, raise the appropriate signal.

PR_SET_PDEATHSIG can be circumvented by a subsequent call unless it is blocked by seccomp, blocked by an LSM (like Yama), or bubblewrap is not dumpable and no other process in the same or child PID namespace has CAP_SYS_PTRACE. Bubblewrap should probably mark itself as non-dumpable to protect against the last scenario.