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.

lisp/emacs-lisp/bytecomp.el

@@ -1183,7 +1183,29 @@ Each function's symbol gets added to `byte-compile-noruntime-functions'."
 	   (compilation-forget-errors)
 	   pt))))
 
+(defvar byte-compile-log-warning-function
+  #'byte-compile--log-warning-for-byte-compile
+  "Function called when encountering a warning or error.
+Called with arguments (STRING POSITION FILL LEVEL).  STRING is a
+message describing the problem.  POSITION is a buffer position
+where the problem was detected.  FILL is a prefix as in
+`warning-fill-prefix'.  LEVEL is the level of the
+problem (`:warning' or `:error').  POSITION, FILL and LEVEL may be
+nil.")
+
 (defun byte-compile-log-warning (string &optional fill level)
+  "Log a byte-compilation warning.
+STRING, FILL and LEVEL are as described in
+`byte-compile-log-warning-function', which see."
+  (funcall byte-compile-log-warning-function
+           string byte-compile-last-position
+           fill
+           level))
+
+(defun byte-compile--log-warning-for-byte-compile (string &optional
+                                                          _position
+                                                          fill
+                                                          level)
   "Log a message STRING in `byte-compile-log-buffer'.
 Also log the current function and file if not already done.  If
 FILL is non-nil, set `warning-fill-prefix' to four spaces.  LEVEL