We treat Item_param whose value is not set as non-const.
This allows us to avoid use of Item_param's value (not yet existing) in 
those fix_fields and fix_length_and_dec that do calculations if their 
Items arguments are const. So we can call fix_fields for such items from
mysql_prepare safely.


sql/item.cc:
  Now Item_param is non-constant (const_item()==FALSE) until its value is set.
sql/item.h:
  Added Item::const_during_execution() method which indicates constants that will be known 
  during execution phase (but they may be not known during preparing phase for example parameters 
  of prep. statements.)
  Made Item_param non-constant until its is value set, so its value won't be requested during 
  prepare statement step.
sql/item_func.cc:
  Fulltext search AGAINST clause now allows prepared statement parameter as its argument.
  Removed duplicate used_tables_cache update in Item_func_match::fix_fields()
  (it is set during Item_func::fix_fields).
tests/client_test.c:
  Added test for bug #1500 "Server crash with mysql_prepare"