Allow specifying the dir locals file to edit (Bug#66663)

Allow specifying the dir locals file to edit (Bug#66663)

* lisp/files-x.el (modify-dir-local-variable): Take a 5th optional
argument, the filename of the dir locals file to modify.
(read-dir-locals-file): New function.
(add-dir-local-variable, delete-dir-local-variable)
(copy-file-locals-to-dir-locals): Optionally read the filename to
modify, and pass it to modify-dir-local-variable.
* etc/NEWS: Announce the change.
* doc/emacs/custom.texi (Directory Variables): Document the new
functionality.