Accept private image_source as set in the standard.

[garloff]

We do produce an info if verbose mode -v is used, as this is of course not desirable, so it's worth a note.

[garloff]

This caused trouble with the VyOS 1.4 rolling 20221121 image on PCO (prod1, prod2) which has the image_source value set to private.
While not preferred, it's legit according to scs-0102-v1, so we have to accept it.

[fkr]

I do think doing grammar / spelling fixes should be a separate commit from a change that changes the logic (since when backing out the functional change also spelling and grammar fixes are lost).

[garloff]

Right.
I do see these things when I look at the code and can't stop myself from fixing.
You are absolutely right, they should be in a different commit.
For larger changes, I'm more disciplined, but I have to admit that I tend to do these smaller fixes along the way as it saves me some time. Sorry for that!