Speed and code space optimziation:

- Cache variables.lc_messages->errmsgs->errmsgs in variables.errmsgs

This gives us 15 byte less code space and 2 memory references for any access to language dependent message,
of which there are 500 in the server..

sql/sql_class.h

@@ -644,6 +644,8 @@ typedef struct system_variables
 
   /* Error messages */
   MY_LOCALE *lc_messages;
+  const char **errmsgs;             /* lc_messages->errmsg->errmsgs */
+
   /* Locale Support */
   MY_LOCALE *lc_time_names;