Initial commit. Allow wanted fontification of open string in any mode.

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.