Add new command checkdoc-package-keywords

Add new command checkdoc-package-keywords

* lisp/emacs-lisp/checkdoc.el (checkdoc-package-keywords-flag): New
  defcustom.
(checkdoc-list-of-strings-p): Add doc.
(checkdoc-current-buffer): When `checkdoc-package-keywords-flag' is
non-nil, call `checkdoc-package-keywords'.
(checkdoc-get-keywords): New defun.
(checkdoc-package-keywords): New command. Warns if the current file
has package.el-style keywords that aren't in `finder-known-keywords'.

* etc/NEWS: Add entry.