Initial commit. Allow wanted fontification of open string in any mode.
The wanted fontification is for the string face to end at the first unescaped newline. This is achieved by a new syntax flag `s' on NL, which means "terminate any open string". src/syntax.c (SYNTAX_FLAGS_CLOSE_STRING, back_maybe_string): New functions. (Fstring_to_syntax, Finternal_describe_syntax_value, scan_lists) (scan_sexps_forward): Adapt to handle the `s' flag. lisp/font-lock.el (font-lock-warn-open-string): New defcustom. (font-lock-fontify-syntactically-region): Enhance to fontify " with warning-face. lisp/progmodes/sh-script.el (sh-mode-syntax-table): Add flag `s' to syntax entry for \n.