Patch on unsupported AT command

[Denis Kenzior]

Hi Marcel,

> I think this might need a GAtChat quirk function where we can add extra
> terminator responses that will be recognized. And maybe even translated
> into something meaningful.

Originally I had the terminators freely definable on the GAtChat + some 
hardcoded ones, but abandoned that approach since it didn't seem useful.  
Perhaps this needs to be resurrected.

>
> Denis, or do you want this quirked in every plugin or modem driver?

I do prefer non-standard terminators to be setup in the plugin/driver.

Regards,
-Denis