Make search for \begin and \end case sensitive.

* latex.el (LaTeX-current-environment): Make search for `\begin'
and `\end' case sensitive.
(docTeX-in-macrocode-p): Ditto.
(LaTeX-indent-calculate): Ditto.
(LaTeX-find-matching-end): Ditto.
(LaTeX-find-matching-begin): Ditto.