Make Ctrl-D delete a setting, because the Text User Interface (tui)
previously provided no way to delete a setting.  Also, update the
on-screen instructions to describe the new feature.  Deleting settings
is especially important for settings stored in precious nonvolatile
storage.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>