Update emacs directory-local variables

- fix C mode
- add C++ mode
- add Octave mode

.dir-locals.el

-((nil .
-      ((c-file-style . "gnu")
-       (indent-tabs-mode . nil)
-       (fill-column . 79)))
- (c-mode . (c-basic-offset 2))
- (makefile-mode . ((indent-tabs-mode . t))))
+((nil . ((indent-tabs-mode . nil)
+         (fill-column . 79)))
+ (c-mode . ((c-file-style . "gnu")))
+ (c++-mode . ((c-file-style . "gnu")))
+ (makefile-mode . ((indent-tabs-mode . t)))
+ (octave-mode . ((octave-block-offset . 4))))