Bug 914753: Make Emacs file variable header lines correct, or at least consistent. DONTBUILD r=ehsan

The -*- file variable lines -*- establish per-file settings that Emacs will
pick up. This patch makes the following changes to those lines (and touches
nothing else):

- Never set the buffer's mode.

Years ago, Emacs did not have a good JavaScript mode, so it made sense
to use Java or C++ mode in .js files. However, Emacs has had js-mode for
years now; it's perfectly serviceable, and is available and enabled by
default in all major Emacs packagings.

Selecting a mode in the -*- file variable line -*- is almost always the
wrong thing to do anyway. It overrides Emacs's default choice, which is
(now) reasonable; and even worse, it overrides settings the user might
have made in their '.emacs' file for that file extension. It's only
useful when there's something specific about that particular file that
makes a particular mode appropriate.

- Correctly propagate settings that establish the correct indentation
level for this file: c-basic-offset and js2-basic-offset should be
js-indent-level. Whatever value they're given should be preserved;
different parts of our tree use different indentation styles.

- We don't use tabs in Mozilla JS code. Always set indent-tabs-mode: nil.
Remove tab-width: settings, at least in files that don't contain tab
characters.

- Remove js2-mode settings that belong in the user's .emacs file, like
js2-skip-preprocessor-directives.