Lots of small cleanups

- Simplified use_trans_cache() to return at once if is_transactional is set
- Indentation and spelling errors fixed
- Don't call signal_update() if update_binlog_end_pos() is called as the
  function already calls signal_update()
- Removed not used function wait_for_update_bin_log(), which would cause
  errors if ever used.
- Simplified handler::clone() by always allocating 'ref' in ha_open(). To do
  this I added an optional MEM_ROOT argument to ha_open() to be used when
  allocating 'ref'
- Changed arguments to get_system_var() from LEX_CSTRING to LEX_CSTRING*
- Added THD as argument to create_select_for_variable(). Changed also char*
  argument to LEX_CSTRING to avoid strlen() call.
- Change calls to append() to use LEX_CSTRING

sql/sql_acl.cc

@@ -8381,18 +8381,18 @@ static void add_user_parameters(String *result, ACL_USER* acl_user,
     {
       DBUG_ASSERT(acl_user->salt_len);
       result->append(STRING_WITH_LEN(" IDENTIFIED BY PASSWORD '"));
-      result->append(acl_user->auth_string.str, acl_user->auth_string.length);
+      result->append(&acl_user->auth_string);
       result->append('\'');
     }
   }
   else
   {
     result->append(STRING_WITH_LEN(" IDENTIFIED VIA "));
-    result->append(acl_user->plugin.str, acl_user->plugin.length);
+    result->append(&acl_user->plugin);
     if (acl_user->auth_string.length)
     {
       result->append(STRING_WITH_LEN(" USING '"));
-      result->append(acl_user->auth_string.str, acl_user->auth_string.length);
+      result->append(&acl_user->auth_string);
       result->append('\'');
     }
   }