bug#38835: 27; Recreated *scratch* buffer has lexical-binding = nil

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 31 Dec 2019 16:52:18 +0100
> 
> The standard *scratch* buffer is blessed with lexical-binding = t, but if 
> killed and recreated via C-x b *scratch* RET, the new buffer isn't.
> 
> Not sure where this logic belongs; the *scratch* handling seems a bit spread 
> out. In Fset_buffer_major_mode?

Maybe we should have a command to recreate *scratch*, and call it from
startup.el?  Then that command could do this.

Or maybe we should decide that lisp-interaction-mode should do this
for any buffer?