nsh with CLE and background tasks

Trying to test the behaviour of CLE with the new code for character position.
I tried this command:

```
nsh>echo "abcd" &
```
to write output in the background and observe the cursor position for CLE.

However this does not really behave as expected and I cant explain what I see;
```
nsh> echo "abcd" &
echo [29:100]
nsh> h_child: BG echo
[6nabcd
child: BG echo complete

nsh: nsh_session: cle failed: 22
nsh>
```

Can you please investigate this? you can reproduce it directly in nsh with a simle "echo".

Configuring readline instead of CLE does not show this "lock" behaviour:
```
nsh> echo abcd &
echo [13:100]
nsh> h_child: BG echo
[Kabcd
child: BG echo complete

nsh>
```