Hook Flymake onto proper checkdoc and byte-compile interfaces

Hook Flymake onto proper checkdoc and byte-compile interfaces

The interfaces in bytecomp.el and checkdoc.el are mostly boilerplate,
with little knowledge of actual internals or thought given to the
usefulness of said interfaces in contexts other than Flymake's.

* lisp/emacs-lisp/bytecomp.el
(byte-compile-log-warning-function): New variable.
(byte-compile-log-warning): Use it.
(byte-compile--log-warning-for-byte-compile): New function.

* lisp/emacs-lisp/checkdoc.el
(checkdoc-create-error-function): New variable.
(checkdoc-create-error): Use it.
(checkdoc--create-error-for-checkdoc): New function.xo

* lisp/progmodes/flymake-elisp.el (flymake-elisp--checkdoc-1):
Use checkdoc-create-error-function.
(flymake-elisp--batch-byte-compile): Use
byte-compile-log-warning-function.