of system vars at PREPARE time": implement a special Item
to handle system variables. This item substitutes itself with 
a basic constant containing variable value at fix_fields.


mysql-test/r/ps.result:
  - test results fixed (Bug#9359).
mysql-test/t/ps.test:
  - add a test case for Bug#9359 "Prepared statements take snapshot
   of system vars at PREPARE time"
sql/item_func.cc:
  - implement Item_func_get_system_var: we should not evaluate system
  variables in the parser, but instead should create an item which 
  is evaluated to a constant at execute.
  - remove an unused function
sql/item_func.h:
  Add a new item, Item_func_get_system_var
sql/mysql_priv.h:
  Move necessary declarations to make set_var.h objects visible in 
  item_func.h
sql/set_var.cc:
  - we should not print to network from get_system_var: if it's called
  from prepared statement prepare, we get packets out of order when using
  the binary protocol. Instead report the error to be sent to the user later.
  This is a backport from 5.0.
sql/set_var.h:
  - declaration of enum_var_type moved to mysql_priv.h