Interface changes pushed early.
  Separation of public and implementation interface for external_lock()
  in preparation for implementation.


sql/handler.cc:
  Separating public virtual external_lock() into public non-virtual
  ha_external_lock() and private virtual external_lock().  Some minor
  changes to ha_*_row() function.
sql/handler.h:
  Separating public virtual external_lock() into public non-virtual
  ha_external_lock() and private virtual external_lock().
sql/lock.cc:
  Using new public ha_external_lock instead of internal external_lock.
sql/opt_range.cc:
  Using new public ha_external_lock instead of internal external_lock.
sql/sql_table.cc:
  Using new public ha_external_lock instead of internal external_lock.