Make `C-h b' indentation more regular (and avoid continuation lines)

Make `C-h b' indentation more regular (and avoid continuation lines)

* lisp/help.el (help--describe-command): Don't do any indentation.
(describe-map): Store data about each section.
(describe-map--align-section): New function to do indentation on a
per-block basis.
(describe-map--fill-columns): Helper function.