emacs-lock-unlockable-modes should default to nil

[Tino Calancha]

Dear Emacs,

I see the comment in the source code: the current value is due
to compatibility with older versions.

I was bitten today with this.  I first thought it was a
bug; then, I realized is the result of a default setting.

Many users would assume that their buffers are secured once they
turn on this minor-mode, overlooking that this option does exist.
I think the value nil is a better default to support such expectations.

Tino