fix to check the existence and the validity of /dev/input/event*

[Terminus-IMRC]

When the raspi-config init script checks whether the shift key is being pushed or not, these 2 problems may occur:

• If the keyboard is not connected, "/dev/input/event" points no file. So thd prints "Error opening '/dev/input/event': No such file or directory".
• If the keyboard had once been connected and is no longer connected, the event device is not valid. So thd prints "Error opening '/dev/input/event0': No such device or address", for example.

So I write a function which checks the existence and the validity of /dev/input/event*. Please evaluate my codes and merge to your master branch.

Regards.

[a-detiste]

That seems a bit much, why not just add a 2&>1 like in my P.R. ? https://github.com/asb/raspi-config/pull/50