Job status not updated on SIGSTOP/SIGCONT

```
build/mrsh -mb
$ sleep 100&
$ kill -STOP $(pidof sleep)
$ jobs
[1] + Running sleep 100 &
$ kill $(pidof sleep)
$
[1] + Done(33024) sleep 100 &
```
The problem seems to be here:
https://github.com/emersion/mrsh/blob/280fdf5ec59d6e66541d6b90e93ad0a3866b58ee/shell/job.c#L228

This check doesn't make sense to me, I would think you would only want to check `WUNTRACED` for children, since checking it for the parent shell process seems useless, but maybe I'm missing something. Also in order to detect SIGCONT the option `WCONTINUED` would need to be used where available, something like:
```
#ifdef WCONTINUED
options |= WUNTRACED | WCONTINUED;
#else
options |= WUNTRACED;
#endif
```
Although maybe that check is too pedantic, I'm not sure if any major platforms lack support for `WCONTINUED` today.

Happy to make a PR but I feel like I don't understand the intention of the check unless `priv->child` is supposed to mean something different than I'm thinking, is it only supposed to be set for grandchildren?