(checkdoc-minor-mode) <defvar>: Remove.

(checkdoc-minor-mode) <defvar>: Remove.
(checkdoc-minor-mode-map): New map, replaces checkdoc-minor-keymap.
(checkdoc-minor-keymap): Backward compatibility.
(checkdoc-minor-menu): Don't bother checking checkdoc-minor-keymap.
(checkdoc-minor-mode) <defun>: Use easy-mmode-define-minor-mode.
(checkdoc-this-string-valid-engine): Be a bit more strict
to avoid matching substrings of `...' quoted vars/funs.
(checkdoc-defun-info): Only look for `interactive' if alone.
(debug-ignored-errors): Add "arg doesn't appear in docstring".