localcharset: optimize code for native Windows.

[Bruno Haible]

The localcharset code for native Windows first fetches
setlocale (LC_ALL, NULL), then notices "oh, this is not what we want",
then fetches setlocale (LC_CTYPE, NULL).

Example:

LC_CTYPE   => "English_United States.1252"
LC_NUMERIC => "French_France.1252"
LC_TIME    => "German_Germany.1252"
LC_ALL     => "LC_COLLATE=English_United States.1252;LC_CTYPE=English_United 
States.1252;LC_MONETARY=English_United 
States.1252;LC_NUMERIC=French_France.1252;LC_TIME=German_Germany.1252"

This can be optimized.


2019-12-17  Bruno Haible  <br...@clisp.org>

        localcharset: Optimize code for native Windows.
        * lib/localcharset.c (locale_charset): Don't both calling
        setlocale (LC_ALL, NULL) since we're not interested in its result.

diff --git a/lib/localcharset.c b/lib/localcharset.c
index 8d17e4c..126f085 100644
--- a/lib/localcharset.c
+++ b/lib/localcharset.c
@@ -901,27 +901,20 @@ locale_charset (void)
      'setlocale' call specified.  So we use it as a last resort, in
      case the string returned by 'setlocale' doesn't specify the
      codepage.  */
-  char *current_locale = setlocale (LC_ALL, NULL);
-  char *pdot;
+  char *current_locale = setlocale (LC_CTYPE, NULL);
+  char *pdot = strrchr (current_locale, '.');
 
-  /* If they set different locales for different categories,
-     'setlocale' will return a semi-colon separated list of locale
-     values.  To make sure we use the correct one, we choose LC_CTYPE.  */
-  if (strchr (current_locale, ';'))
-    current_locale = setlocale (LC_CTYPE, NULL);
-
-  pdot = strrchr (current_locale, '.');
   if (pdot && 2 + strlen (pdot + 1) + 1 <= sizeof (buf))
     sprintf (buf, "CP%s", pdot + 1);
   else
     {
       /* The Windows API has a function returning the locale's codepage as a
-        number: GetACP().
-        When the output goes to a console window, it needs to be provided in
-        GetOEMCP() encoding if the console is using a raster font, or in
-        GetConsoleOutputCP() encoding if it is using a TrueType font.
-        But in GUI programs and for output sent to files and pipes, GetACP()
-        encoding is the best bet.  */
+         number: GetACP().
+         When the output goes to a console window, it needs to be provided in
+         GetOEMCP() encoding if the console is using a raster font, or in
+         GetConsoleOutputCP() encoding if it is using a TrueType font.
+         But in GUI programs and for output sent to files and pipes, GetACP()
+         encoding is the best bet.  */
       sprintf (buf, "CP%u", GetACP ());
     }
   /* For a locale name such as "French_France.65001", in Windows 10,