(latex-mode-map): Bind latex-split-block to C-c C-s.

(latex-mode-map): Bind latex-split-block to C-c C-s.
(tex-font-lock-keywords-2): Comment out unused var `type'.
(tex-guess-mode): New name for old `tex-mode'.
(tex-mode): Make it a derived mode of text-mode.
(plain-tex-mode, latex-mode): Change parent from text-mode to tex-mode.
(tex-common-initialization): Don't setup syntax-table any more.
(tex-validate-buffer): Remove unused var `oend'.
Use with-current-buffer and line-beginning-position.
(tex-recenter-output-buffer): Remove unused var `old-buffer'.
(latex-indent): Return `noindent' if no indentation can happen.
(doctex-font-lock-^^A, doctex-font-lock-syntactic-face-function)
(doctex-font-lock-syntactic-keywords, doctex-font-lock-keywords)
(doctex-mode): New funs for DocTeX mode, derived from LaTeX mode.