Defaults for elisp-mode files

[Stefan Monnier]

I'm seeing more and more Elisp files with "-*- lexical-binding: t;
coding: utf-8 -*-" showing up, and I expect them to become the norm in
the longer term.

So I think in the longer term these annotations should be made
unnecessary because they're ugly.
For the `utf-8', I guess we can simply set auto-coding-alist and we
could do that right now without much risk for serious breakage (IOW
I suggest we do that for the next release after 24.3).

But for the lexical-binding setting, it seems less obvious how to handle
such a switch (other than start warning any file which doesn't set it
explicitly to t or nil, and after a while change the default, and after
yet another while remove the warning, the whole process taking probably
more than 10 years).  Any suggestion would be welcome.


        Stefan