Curved quotes in --batch diagnostics in non-UTF-8

When run with --batch, check that curved quotes are compatible with
the system locale before outputting them in diagnostics.
Problem reported by Eli Zaretskii in:
http://lists.gnu.org/archive/html/emacs-devel/2015-08/msg00594.html
* lisp/startup.el (command-line): Set internal--text-quoting-flag
after the standard display table is initialized.
* src/doc.c (default_to_grave_quoting_style): New function.
(text_quoting_style): Use it.
(text_quoting_flag): New static var, visible to Lisp as
internal--text-quoting-flag.
* src/emacs.c: Include <wchar.h> if available.
(using_utf8): New function.
(main): Use it to initialize text_quoting_flag.
* src/regex.h (btowc) [WIDE_CHAR_SUPPORT && emacs]:
Don't define, as it's not needed and it clashes with wchar.h.

lisp/startup.el

@@ -1023,6 +1023,7 @@ please check its value")
       (or standard-display-table
           (setq standard-display-table (make-display-table)))
       (aset standard-display-table (car char-repl) (cdr char-repl))))
+  (setq internal--text-quoting-flag t)
 
   ;; Re-evaluate predefined variables whose initial value depends on
   ;; the runtime context.