el/dot-emacs.el: Fix tab key in TeX-ish modes.

I think having TAB bound to `indent-for-tab-command' is after all a good
thing, and I should put it back.  But `indent-relative' is also useful,
so put that on M-TAB.  This means defeating Flyspell's use of this key
to mean the same as C-. (`flyspell-auto-correct-word'), which (a) is
already on a different key too, and (b) I didn't know existed so I'm not
going to miss it.