Obey coding-system-for-write when writing stdout/stderr in batch

* src/print.c (printchar_to_stream):
* src/xdisp.c (message_to_stderr): If coding-system-for-write has
a non-nil value, use it to encode output in preference to
locale-coding-system.  See the discussions in
http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00048.html
for the details.

* doc/lispref/os.texi (Terminal Output): Document how to send
non-ASCII text via 'send-string-to-terminal'.
(Batch Mode): Document how text written to standard streams is
encoded.  Fix inaccuracy regarding which output streams are used
by output functions in batch mode.

doc/lispref/os.texi

@@ -2085,6 +2085,8 @@ than optimal.  To fix the problem, set @code{baud-rate}.
 @defun send-string-to-terminal string &optional terminal
 This function sends @var{string} to @var{terminal} without alteration.
 Control characters in @var{string} have terminal-dependent effects.
+(If you need to display non-ASCII text on the terminal, encode it
+using one of the functions described in @ref{Explicit Encoding}.)
 This function operates only on text terminals.  @var{terminal} may be
 a terminal object, a frame, or @code{nil} for the selected frame's
 terminal.  In batch mode, @var{string} is sent to @code{stdout} when
@@ -2252,13 +2254,21 @@ loads the library named @var{file}, or @samp{-f @var{function}}, which
 calls @var{function} with no arguments, or @samp{--eval @var{form}}.
 
   Any Lisp program output that would normally go to the echo area,
-either using @code{message}, or using @code{prin1}, etc., with @code{t}
-as the stream, goes instead to Emacs's standard error descriptor when
-in batch mode.  Similarly, input that would normally come from the
-minibuffer is read from the standard input descriptor.
-Thus, Emacs behaves much like a noninteractive
-application program.  (The echo area output that Emacs itself normally
-generates, such as command echoing, is suppressed entirely.)
+either using @code{message}, or using @code{prin1}, etc., with
+@code{t} as the stream, goes instead to Emacs's standard descriptors
+when in batch mode: @code{message} writes to the standard error
+descriptor, while @code{prin1} and other print functions write to the
+standard output.  Similarly, input that would normally come from the
+minibuffer is read from the standard input descriptor.  Thus, Emacs
+behaves much like a noninteractive application program.  (The echo
+area output that Emacs itself normally generates, such as command
+echoing, is suppressed entirely.)
+
+Non-ASCII text written to the standard output or error descriptors is
+by default encoded using @code{locale-coding-system} (@pxref{Locales})
+if it is non-@code{nil}; this can be overridden by binding
+@code{coding-system-for-write} to a coding system of you choice
+(@pxref{Explicit Encoding}).
 
 @defvar noninteractive
 This variable is non-@code{nil} when Emacs is running in batch mode.

src/print.c

@@ -200,6 +200,13 @@ printchar_to_stream (unsigned int ch, FILE *stream)
 {
   Lisp_Object dv IF_LINT (= Qnil);
   ptrdiff_t i = 0, n = 1;
+  Lisp_Object coding_system = Vlocale_coding_system;
+  bool encode_p = false;
+
+  if (!NILP (Vcoding_system_for_write))
+    coding_system = Vcoding_system_for_write;
+  if (!NILP (coding_system))
+    encode_p = true;
 
   if (CHAR_VALID_P (ch) && DISP_TABLE_P (Vstandard_display_table))
     {
@@ -228,8 +235,11 @@ printchar_to_stream (unsigned int ch, FILE *stream)
 	  unsigned char mbstr[MAX_MULTIBYTE_LENGTH];
 	  int len = CHAR_STRING (ch, mbstr);
 	  Lisp_Object encoded_ch =
-	    ENCODE_SYSTEM (make_multibyte_string ((char *) mbstr, 1, len));
+	    make_multibyte_string ((char *) mbstr, 1, len);
 
+	  if (encode_p)
+	    encoded_ch = code_convert_string_norecord (encoded_ch,
+						       coding_system, true);
 	  fwrite (SSDATA (encoded_ch), 1, SBYTES (encoded_ch), stream);
 #ifdef WINDOWSNT
 	  if (print_output_debug_flag && stream == stderr)

src/xdisp.c

@@ -10206,7 +10206,16 @@ message_to_stderr (Lisp_Object m)
     }
   if (STRINGP (m))
     {
-      Lisp_Object s = ENCODE_SYSTEM (m);
+      Lisp_Object coding_system = Vlocale_coding_system;
+      Lisp_Object s;
+
+      if (!NILP (Vcoding_system_for_write))
+	coding_system = Vcoding_system_for_write;
+      if (!NILP (coding_system))
+	s = code_convert_string_norecord (m, coding_system, true);
+      else
+	s = m;
+
       fwrite (SDATA (s), SBYTES (s), 1, stderr);
     }
   if (!cursor_in_echo_area)