Use emacs 25's checkdoc-package-keywords and checkdoc-file in build

[Trevoke]

Changes in Specialized Modes and Packages in Emacs 25.1

New command 'checkdoc-package-keywords' checks if the
current package keywords are recognized. Set the new option
'checkdoc-package-keywords-flag' to non-nil to make
'checkdoc-current-buffer' call this function automatically.

New function 'checkdoc-file' checks for style errors.
It's meant for use together with 'compile':
emacs -batch --eval "(checkdoc-file "subr.el")"