my_init: handle OSError from waitpid(-1, 0).

It's possible for all processes running under my_init to exit and have
been reaped before waitpid_reap_other_children calls wait(-1, 0).  This
is especially likely with `--skip-runit` and a simple process like
`true` being run.

Here's an example real-world stack trace:

    docker run --rm image_name /sbin/my_init --quiet --skip-runit -- true

    Traceback (most recent call last):
      File "/sbin/my_init", line 312, in <module>
        main(args)
      File "/sbin/my_init", line 238, in main
        run_startup_files()
      File "/sbin/my_init", line 193, in run_startup_files
        run_command_killable_and_import_envvars(filename)
      File "/sbin/my_init", line 156, in run_command_killable_and_import_envvars
        run_command_killable(*argv)
      File "/sbin/my_init", line 143, in run_command_killable
        status = waitpid_reap_other_children(pid)
      File "/sbin/my_init", line 109, in waitpid_reap_other_children
        this_pid, status = os.waitpid(-1, 0)
    OSError: [Errno 10] No child processes

Here's the same issue boiled down to a one-liner:

    $ python -c 'import os; os.waitpid(-1, 0)'
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    OSError: [Errno 10] No child processes