How to use -list-target-features MI command

[Jan Vrany <jan dot vrany at fit dot cvut dot cz>]

Hi there, 

I need to know (in advance) whether the target supports async 
mode or not. 
I know that -list-target-features can be used for that, but
not sure when and how to use -list-target-features. 

The documentation says:

"Whenever a target can change, due to commands such as -target-select,
-target-attach or -exec-run, the list of target features may change,
and the frontend should obtain it again."

However, once I issue `-exec-run` using a target with no async mode
support, I will not get the result of -list-target-feature before
debugee stops. 

Another problem is that user may enter CLI command manually. In case,
how do I find out that target may have changed and I should check for
features? 

Could you give me any advice on how and when to correctly check for
target features? 

Thanks a lot! 

Jan