lock table for writing during prepare of statement.

When single call open_normal_and_derived_tables() was used, we never
set table_count to the right value.  This patch reverts the part of
the old code that does open_tables() (and sets table_count), then
checks if table_list->multitable_view is set (and returns if so, using
table_count value), and only then it does mysql_handle_derived().