(tex-font-lock-keywords-1): Use `keep'

(tex-font-lock-keywords-1): Use `keep'
rather than `prepend' and add an interesting comment.
(tex-math-face, tex-font-lock-syntactic-face-function):
New face and function to use it.
(tex-define-common-keys, tex-mode-map): Use menu-item rather
than `menu-enable' symbol property.
(tex-mode-map): Bind {, (, [ and $ to skeleton-pair-insert-maybe.
(tex-mode): Add some latex-mode commands for auto-selection.
Use tex-font-lock-syntactic-face-function.
(tex-insert-quote): Simplify.
(tex-shell): New mode.
(tex-start-shell): Use it.
(tex-shell-proc, tex-shell-buf): New functions.
(tex-send-command): Use it.
(tex-main-file): Fix the meaning of the new arg REALFILE.
(tex-send-tex-command): New function split from `tex-start-tex'.
Set compilation-last-buffer and compilation-parsing-end.