Add response to ESCAPE key, as advertised in the documentation.

[cjungmann]

I'm not even sure if you accept pull requests, since don't seem to be any.

I couldn't get the ESCAPE key to exit whiptail forms, so I found the omission and added two lines of code.

[mlichvar]

Your change would cause all applications (not just whiptail) to react to ESC.

A better way would be to add ESC as a hotkey in whiptail. It should probably be limited to dialogs that have the cancel button, otherwise I suspect it might lead to unexpected behavior.

Another option is to fix the man page to not mention the escape key.