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Make it into a simple syntax-table, shared among all submodes.
(sh-heredoc-face): Re-introduce.
(sh-font-lock-syntactic-face-function): New function.
(sh-mode): Use it.  Also use define-derived-mode.
Remove old bogus setting of indent-region-function.
(sh-set-shell): Don't set the syntax-table any more.
(sh-mode-syntax-table) <defun>: Remove.