daemon: Use G_SPAWN_STDOUT_TO_DEV_NULL as required.

The g_spawn_sync documentation says:

    Note that you must set the G_SPAWN_STDOUT_TO_DEV_NULL and
    G_SPAWN_STDERR_TO_DEV_NULL flags when passing NULL for
    standard_output and standard_error.

And indeed, without this, "shutdown" gets a EPIPE when writing to
stdout, which is a harmless but distracting error.

Closes #683

Reviewed-by: Stef Walter <stefw@redhat.com>