don't evaluate subqueries during statement prepare, even if they
are not correlated.
With post-review fixes.


sql/mysql_priv.h:
  Add UNCACHEABLE_PREPARE to mark subqueries as non-constant in 
  mysql_stmt_prepare
sql/sql_lex.cc:
  Add a missing assert: noone can call unit::set_limit from 
  mysql_stmt_prepare.
sql/sql_lex.h:
  Comment fixed.
sql/sql_parse.cc:
  Mark new SELECT_LEXes as uncacheable if they created during 
  statement prepare.
sql/sql_prepare.cc:
  Switch off the uncacheable flag when prepare is done.