Add unattended spell-checking to checkdoc

Add unattended spell-checking to checkdoc

This commit makes checkdoc capable of spell-checking even when the
user isn't using it interactively. When TAKE-NOTES is non-nil,
checkdoc will run spell-checking (with ispell) and report spelling
mistakes.

Fixes: (bug#38583).
* lisp/textmodes/ispell.el (ispell-word): Extract part of it to
`ispell--run-on-word`.
(ispell--run-on-word): New function, extracted from `ispell-word`.
(ispell-error-checking-word): New function.
(ispell-correct-p): New function.  Use `ispell--run-on-word` and
`ispell-error-checking-word`.
* lisp/emacs-lisp/checkdoc.el (checkdoc-current-buffer): Pass
TAKE-NOTES to `checkdoc-start`.
(checkdoc-continue): Pass TAKE-NOTES to `checkdoc-this-string-valid`.
(checkdoc-this-string-valid): Add optional argument TAKE-NOTES and
pass it to `checkdoc-this-string-valid-engine`.
(checkdoc-this-string-valid-engine): Add optional argument TAKE-NOTES
and pass it to `checkdoc-ispell-docstring-engine`.
(checkdoc-ispell-init): Call `ispell-set-spellchecker-params` and
`ispell-accept-buffer-local-defs`.  These calls are required to
properly use ispell.  The problem went unnoticed until now because
checkdoc was only using ispell through the high-level command
`ispell-word` which takes care of all the initialization for the user.
(checkdoc-ispell-docstring-engine): Add optional argument TAKE-NOTES
to force reporting of spell-checking errors.  Throw error
when (checkdoc-ispell-init) fails configuring ispell.  Replace a
few (if cond nil body) with (unless cond body). Replace (let ((var
nil))) with (let (var)). Replace (if (not (eq checkdoc-autofix-flag
'never)) body) with just body because `checkdoc-autofix-flag` is
checked at the beginning of the function.