key_defined - check if a keycode is defined
int key_defined(const char *definition);
This is an extension to the curses library. It permits an application to
determine if a string is currently bound to any keycode.
If the string is bound to a keycode, its value (greater than zero) is returned.
If no keycode is bound, zero is returned. If the string conflicts with longer
strings which are bound to keys, -1 is returned.
These routines are specific to ncurses. They were not supported on Version 7,
BSD or System V implementations. It is recommended that any code depending on
them be conditioned using NCURSES_VERSION.