Improve CFEngine 3 parsing and eldoc support, with syntax fallbacks

Improve CFEngine 3 parsing and eldoc support, with syntax fallbacks

* progmodes/cfengine.el (cfengine-cf-promises): Add more default
locations for cf-promises.
(cfengine-mode-syntax-functions-regex): New caching variable.
(cfengine3-fallback-syntax): Fallback syntax for cases where
cf-promises doesn't run.
(cfengine3--current-word): Reimplement using
`cfengine-mode-syntax-functions-regex'.
(cfengine3-completion-function, cfengine3--current-function): Use
`cfengine3-make-syntax-cache' directly.
(cfengine3-clear-syntax-cache): New function.
(cfengine3-make-syntax-cache): Simplify and create
`cfengine-mode-syntax-functions-regex' on demand.
(cfengine3-format-function-docstring): Don't call
`cfengine3-make-syntax-cache' explicitly.