For curses display, recognize a few more control keys

The curses display handles most control-X keys, and translates
them into their corresponding keycode.  Here we recognize
a few that are missing, Ctrl-@ (null), Ctrl-\ (backslash),
Ctrl-] (right bracket), Ctrl-^ (caret), Ctrl-_ (underscore).

Signed-off-by: Sean Estabrooks <sean.estabrooks@gmail.com>
Message-id: CAHyVn3Bh9CRgDuOmf7G7Ngwamu8d4cVozAcB2i4ymnnggBXNmg@mail.gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>