cfengine.el support and updates

[Ted Zlatanov]

I have a PR against core.git to update cfengine.el:

https://github.com/cfengine/core/pull/2015

With this PR, I am officially dropping support for Emacs 23 and older
and XEmacs, unless someone else is willing to test and maintain the
compatibility code. It's just not worth the effort, since Emacs 24.1 was
released in 2012 and keeping things up to date with the official version
in the Emacs trunk is a pain. Ideally I'd bring cfengine.el even closer
to the official version and minimize the maintenance burden; then I can
work on more interesting features like context-sensitive completion
popups and smarter indentation.

The PR also adds a few other nice things like Imenu and prettify-symbols
support.

Thanks
Ted