Fixed "discover" in the handler API.

Fixed problem where handlerton was not zero'ed. I need to look around, I suspect this problem is more widespread. 


sql/ha_innodb.h:
  Unused variable
sql/ha_ndbcluster.cc:
  Added "discover" to handlerton.
sql/handler.cc:
  Added plugin loop and correctly now use handler API.
sql/handler.h:
  Removed unused variable. Added discover to handler API
sql/mysqld.cc:
  Removed unused variables.
sql/sql_plugin.cc:
  Fixed DBUG Enter comment (obvious cut paste mistake)
storage/csv/ha_tina.cc:
  Found that if we don't bzero handlerton, that things can go boom!
  
  This probably needs to be fixed for all handlers