BUG#4286 - HANDLER tables are closed by FLUSH TABLE(S).

BUG#4335 - one name can be handler open'ed many times.
Reworked the HANDLER functions and interface.
Using a HASH to store information on open tables that
survives FLUSH TABLE.
HANDLER tables alias names must now be unique, though it
is allowed in 4.0 to qualify them with the database name
of the base table.

sql/sql_class.h

@@ -421,6 +421,7 @@ public:
      and are still in use by this thread
   */
   TABLE   *open_tables,*temporary_tables, *handler_tables;
+  HASH    handler_tables_hash;
   // TODO: document the variables below
   MYSQL_LOCK *lock,*locked_tables;
   ULL	  *ull;