ReadKey function

  DECLARATION:  ReadKey

      PURPOSE:  Reads a character from the keyboard

         UNIT:  CRT

      REMARKS:  A single character is read from the keyboard without
                echoing it to the screen.  If KeyPressed was TRUE before
                the call to ReadKey, the character is returned immediately,
                else ReadKey will wait for a key to be pressed.

                If a special key is pressed (Function keys, cursor movement,
                Alt keys, etc.) ReadKey will first return the Null character,
                (#0), then it will return the extended key scan code.

                NOTE: When the system variable, CheckBreak, is FALSE, ReadKey
                      will return a CTRL-C (#3) when CTRL-Break is pressed.