MDEV-27525 Invalid (non-UTF8) characters found for option 'plugin_dir'

Two Problems
1. Upgrade wizard failed to retrieve path to service executable,
if it contained non-ASCII.
Fixed by setlocale(LC_ALL, "en_US.UTF8"), which was missing in upgrade wizard

2.mysql_upgrade_service only updated (converted to UTF8) the server's sections
leaving client's as-is

Corrected typo.

3. Fixed assertion in my_getopt, turns out to be too strict.

mysys/my_getopt.c

@@ -163,11 +163,6 @@ static void validate_value(const char *key, const char *value,
   }
   else
   {
-    /*
-     Should never happen, non-UTF8 can be read from option's
-     file only.
-    */
-    DBUG_ASSERT(0);
     my_getopt_error_reporter(
     WARNING_LEVEL, "%s: invalid (non-UTF8) characters for option %s",
       my_progname, key);