updated emacs configuration

I think the suggested emacs configuration snippets in
src/tools/editors/emacs.samples no longer represent current best
practices.  I have come up with some newer things that I'd like to
propose for review.

First, I propose adding a .dir-locals.el file to the top-level directory
with basic emacs settings.  These get applied automatically.  This
especially covers the particular tab and indentation settings that
PostgreSQL uses.  With this, casual developers will not need to modify
any of their emacs settings.

(In the attachment, .dir-locals.el is called _dir-locals.el so that it
doesn't get lost.  To clarify, it goes into the same directory that
contains configure.in.)

With that, emacs.samples can be shrunk significantly.  The only real
reason to keep is that that c-offsets-alist and (more dubiously)
sgml-basic-offset cannot be set from .dir-locals.el because they are not
"safe".  I have also removed many of the redundant examples and settled
on a hook-based solution.

I think together this setup would be significantly simpler and more
practical.