MDEV-26713 UTF8 support on Windows, convert my.ini from ANSI to UTF8

Handle upgrade - on Windows that is capable of UTF8 codepage, convert
entries in my.ini from ANSI to UTF8, during upgrade

Reason is that for the server, paths such as datadir, innodb directories,
location of SSL certificates must now be utf8.

For the client programs , user name,  database etc should be in UTF8,
too, as UTF-8 is now the default charset.

sql/upgrade_conf_file.cc

@@ -25,6 +25,8 @@
   Note : the list below only includes the default-compiled server and none of the
   loadable plugins.
 */
+#include <my_global.h>
+#include <my_sys.h>
 #include <windows.h>
 #include <initializer_list>
 #include <stdlib.h>
@@ -161,11 +163,52 @@ static int cmp_strings(const void* a, const void *b)
 
 #define MY_INI_SECTION_SIZE 32 * 1024 + 3
 
+static bool is_utf8_str(const char *s)
+{
+  MY_STRCOPY_STATUS status;
+  const struct charset_info_st *cs= &my_charset_utf8mb4_bin;
+  size_t len= strlen(s);
+  if (!len)
+    return true;
+  cs->cset->well_formed_char_length(cs, s, s + len, len, &status);
+  return status.m_well_formed_error_pos == nullptr;
+}
+
+
+static UINT get_system_acp()
+{
+  static DWORD system_acp;
+  if (system_acp)
+    return system_acp;
+
+  char str_cp[10];
+  int cch= GetLocaleInfo(GetSystemDefaultLCID(), LOCALE_IDEFAULTANSICODEPAGE,
+                         str_cp, sizeof(str_cp));
+
+  system_acp= cch > 0 ? atoi(str_cp) : 1252;
+
+  return system_acp;
+}
+
+
+static char *ansi_to_utf8(const char *s)
+{
+#define MAX_STR_LEN MY_INI_SECTION_SIZE
+  static wchar_t utf16_buf[MAX_STR_LEN];
+  static char utf8_buf[MAX_STR_LEN];
+  if (MultiByteToWideChar(get_system_acp(), 0, s, -1, utf16_buf, MAX_STR_LEN))
+  {
+    if (WideCharToMultiByte(CP_UTF8, 0, utf16_buf, -1, utf8_buf, MAX_STR_LEN,
+                            0, 0))
+      return utf8_buf;
+  }
+  return 0;
+}
 
 int fix_section(const char *myini_path, const char *section_name,
                 bool is_server)
 {
-  if (!is_server)
+  if (!is_server && GetACP() != CP_UTF8)
     return 0;
 
   static char section_data[MY_INI_SECTION_SIZE];
@@ -188,6 +231,21 @@ int fix_section(const char *myini_path, const char *section_name,
       continue;
 
     value= key_end + 1;
+    if (GetACP() == CP_UTF8 && !is_utf8_str(value))
+    {
+      /*Convert a value, if it is not already UTF-8*/
+      char *new_val= ansi_to_utf8(value);
+      if (new_val)
+      {
+        *key_end= 0;
+        fprintf(stdout, "Fixing variable '%s' charset, value=%s\n", keyval,
+                new_val);
+        WritePrivateProfileString(section_name, keyval, new_val, myini_path);
+        *key_end= '=';
+      }
+    }
+    if (!is_server)
+      continue;
 
     // Check if variable should be removed from config.
     // First, copy and normalize (convert dash to underscore) to  variable