Commands to add/delete file/directory-local variables.

Commands to add/delete file/directory-local variables.
(read-file-local-variable, read-file-local-variable-value)
(read-file-local-variable-mode, modify-file-local-variable)
(modify-file-local-variable-prop-line)
(modify-dir-local-variable): New functions.
(add-file-local-variable, delete-file-local-variable)
(add-file-local-variable-prop-line, delete-file-local-variable-prop-line)
(add-dir-local-variable, delete-dir-local-variable)
(copy-file-locals-to-dir-locals, copy-dir-locals-to-file-locals)
(copy-dir-locals-to-file-locals-prop-line): New commands.