Make sure that bind-key's `override-global-mode` is initially on

Make sure that bind-key's `override-global-mode` is initially on

In 4004dde the arguments to `define-minor-mode` were changed
erroneously. Whereas the `override-global-mode` was initially defined
as `(define-minor-mode override-global-mode "..." t "")`, the two
latter arguments where changed to `:global t :lighter ""`. However,
the two original arguments corresponded to the keywords `:init-value`
and `:lighter`, respectively.

With `:init-value t` now missing, the minor mode isn't enabled by
default, and `bind-key*` appears not to work.

Copyright-paperwork-exempt: yes