'flush tables' crashes

The server crashes when 'show procedure status' and 'flush tables' are
run concurrently.

This is caused by the way mysql.proc table is added twice to the list
of table to lock although the requirements on the current locking API
assumes differently.

No test case is submitted because of the nature of the crash which is 
currently difficult to reproduce in a deterministic way.

This is a backport from 5.1

myisam/mi_dbug.c:
  * check_table_is_closed is only used in EXTRA_DEBUG mode but since it is
  iterating over myisam shared data it still needs to be protected by an
  appropriate mutex.
sql/sql_yacc.yy:
  * Since the I_S mechanism is already handling the open and close of 
  mysql.proc there is no need for the method sp_add_to_query_tables.