Documentation/CodingStyle: Use directory-local variables for emacs settings

In emacs 23.1 support for directory-local variables was added (see also
https://lists.gnu.org/archive/html/info-gnu-emacs/2009-07/msg00000.html).
Simplify the settings in coding-style.rst by using that feature.
Additionally, do not inherit any settings from emacs' linux coding style
to minimize dependencies on the version of emacs that is being used.

I have verified with several large and nontrivial kernel source files
that the new settings format code according to what checkpatch expects.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Alison Chaiken <alison@she-devel.com>
Cc: Joe Perches <joe@perches.com>
Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Geyslan G. Bem <geyslan@gmail.com>
Cc: Tiago Natel de Moura <tiago4orion@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>