(with-buffer-unmodified): Use unwind-protect.

(with-buffer-unmodified): Use unwind-protect.
(jit-lock-mode): Make sure font-lock-keywords-only is bound before use.
(jit-lock-functions): New var.
(jit-lock-function-1): Use it if non-nil.
Don't switch the syntax-table.  Don't set parse-sexp-lookup-properties.
Set the `fontified' property before doing the fontification to avoid
repeatedly going through the same error.
Don't turn errors into messages.