Scan and monitor examples using libgpiod

[fsheikh]

Summary: This patch adds a simple C example to search for a GPIO line on a Linux platform with multiple GPIO controllers. It also adds another c++ example using event loop to monitor a GPIO line.

[fsheikh]

Hi @starnight. I really found the examples quite useful. Although one can look into ligpiod official project, they are still useful for example when one does not want to rebuilt that whole project. Just added some more examples which fitted a use-case I was working on. All in the hope that others may find them useful. Please feel free to review. Cheers!

[fsheikh]

@starnight please feel free to have another look!

[fsheikh]

After 4 years, I just learned I did not submit the review request.

Perhaps it is never too late :smile:

[starnight]

Squashed the fix into the previous commits and pushed to master. Thanks