Add an example .dir-locals.el

This file, when copied to .dir-locals.el in the OpenSSL source top,
will make sure that the CC mode style "OpenSSL-II" will be used for
all C files.

Additionally, I makes sure that tabs are never used as indentation
character, regardless of the emacs mode, and that the fill column is
78.

Reviewed-by: Rich Salz <rsalz@openssl.org>