VIEWs, Prepared statement": we didn't mark the nested views
as 'prelockng placeholders' when building the prelocking list. 
This resulted in these views being processed (merged, materialized)
before they are actually used. 


mysql-test/r/sp-prelocking.result:
  Test results fixed (Bug#15683)
mysql-test/t/sp-prelocking.test:
  Add a test case for Bug#15683
sql/sql_view.cc:
  When initializing tables used in a view, inherite 
  its 'prelocking_placeholder' flag. This flag is set when we're 
  opening the view in order to build the prelocking list for a query.
  In this case we should only calculate the list to 
  prelock, and should not merge neither this view nor any of the 
  nested views, of there are any. The flag is used to indicate this 
  mode.