MDEV-20728: /usr/sbin/mysqld: unknown variable 'defaults-group-suffix=mysqld1

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.

scripts/mysqld_multi.sh

@@ -372,7 +372,7 @@ sub start_mysqlds()
     if (!$suffix_found)
     {
       $com.= " --defaults-group-suffix=";
-      $com.= $groups[$i];
+      $com.= substr($groups[$i],6);
     }
 
     $com.= $tmp;