Refactor Emacs Lisp checking forms

Don't splice the source file into the check forms, and instead use
Flycheck's standard substitution mechanism to pass the sources as
arguments.

Reset command-line-args-left to avoid #319
Extract common code from emacs lisp and checkdoc checkers

Add flycheck-prepare-emacs-lisp-form which wraps up the common setup
from both checkers.
Simplify predicates of Emacs Lisp checkers